(DOCSP-39503): Consolidate Relationships page (#3287)

## Pull Request Info - SDK Docs Consolidation

Jira ticket: https://jira.mongodb.org/browse/DOCSP-39503

*Staged Page*

-
[Relationships](https://preview-mongodbdacharyc.gatsbyjs.io/realm/DOCSP-39503/sdk/model-data/relationships/)

*Page Source*

Add links to every SDK's pages where you got the SDK-specific
information:

- [C++:
Relationships](https://www.mongodb.com/docs/atlas/device-sdks/sdk/cpp/model-data/relationships/)
- [Flutter:
Relationships](https://www.mongodb.com/docs/atlas/device-sdks/sdk/flutter/realm-database/model-data/relationships/)
- [Java:
Relationships](https://www.mongodb.com/docs/atlas/device-sdks/sdk/java/model-data/relationships/)
- [Kotlin:
Relationships](https://www.mongodb.com/docs/atlas/device-sdks/sdk/kotlin/realm-database/schemas/relationships/)
- [.NET:
Relationships](https://www.mongodb.com/docs/atlas/device-sdks/sdk/dotnet/model-data/relationships/)
- [Node.js: Relationships and Embedded
Objects](https://www.mongodb.com/docs/atlas/device-sdks/sdk/node/model-data/relationships-and-embedded-objects/)
- [Swift:
Relationships](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/model-data/relationships/)

### PR Author Checklist

Before requesting a review for your PR, please check these items:

- [x] Open the PR against the `feature-consolidated-sdk-docs` branch
instead of `master`
- [x] Tag the consolidated page for:
  - genre
  - meta.keywords
  - meta.description

#### Naming
- [x] Update Realm naming and the language around persistence
layer/local/device per [this
document](https://docs.google.com/document/d/126OczVxBWAwZ4P5ZsSM29WI3REvONEr1ald-mAwPtyQ/edit?usp=sharing)
- [x] Include `.rst` files comply with [the naming
guidelines](https://docs.google.com/document/d/1h8cr66zoEVeXytVfvDxlCSsUS5IZwvUQvfSCEXNMpek/edit#heading=h.ulh8b5f2hu9)

#### Links and Refs
- [x] Create new consolidated SDK ref targets starting with "_sdks-" for
relevant sections
- [x] Remove or update any SDK-specific refs to use the new consolidated
SDK ref targets
- [x] [Update any Kotlin API
links](https://jira.mongodb.org/browse/DOCSP-32519) to use the new
Kotlin SDK roles

#### Content
- [x] Shared code boxes have snippets or placeholders for all 9
languages
- [x] API description sections have API details or a generic placeholder
for all 9 languages
- [x] Check related pages for relevant content to include
- [x] Create a ticket for missing examples in each relevant SDK:
Consolidation Gaps epic

### Reviewer Checklist

As a reviewer, please check these items:

- [x] Shared code example boxes contain language-specific snippets or
placeholders for every language
- [x] API reference details contain working API reference links or
generic content
- [x] Realm naming/language has been updated
- [x] All relevant content from individual SDK pages is present on the
consolidated page

Author: dacharyc

source/includes/api-details/cpp/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,7 @@
+To define an inverse relationship, use ``linking_objects`` in your object 
+model. The ``linking_objects`` definition specifies the object type and 
+property name of the relationship that it inverts.
+
+In this example, we define a ``Person`` having a to-one relationship with
+a ``Dog``. The ``Dog`` has an inverse relationship to any ``Person`` 
+objects through its ``owners`` property.

source/includes/api-details/csharp/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,18 @@
+To define the inverse relationship, define a getter-only ``IQueryable<T>``
+property in your object model, where ``T`` is the source type of the
+relationship. Then, annotate this property with a 
+:dotnet-sdk:`[Backlink(sourceProperty)] <reference/Realms.BacklinkAttribute.html>` 
+attribute, where "sourceProperty" is the name of the property on the other 
+side of the relationship. The following example shows how to do this with the 
+"User has many Items" scenario.
+
+In this example, note that:
+
+- The Item object's ``Assignee`` property is a ``User`` object.
+
+- The ``User`` object's ``Items`` property inverts the relationship and 
+  refers to all ``Item`` objects that contain this specific ``User`` in their 
+  ``Assignee`` property. 
+
+This, then, allows us to query the ``Item`` collection to get all ``Items``
+assigned to a specific ``User``.

source/includes/api-details/dart/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,6 @@
+Use the `Backlink <https://pub.dev/documentation/realm_common/latest/realm_common/Backlink-class.html>`__
+property annotation to define an inverse relationship.
+Pass a `Symbol <https://api.dart.dev/stable/2.18.4/dart-core/Symbol/Symbol.html>`__
+of the field name of the to-one or to-many field for which you are creating the
+backlink as an argument to ``Backlink()``. Include an ``Iterable`` of the
+object model you are backlinking to in the field below the annotation.

source/includes/api-details/java/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,9 @@
+To define an inverse relationship, define a ``LinkingObjects`` property in your
+object model. The ``LinkingObjects`` definition specifies the object type and
+property name of the relationship that it inverts.
+
+Fields annotated with ``@LinkingObjects`` must be:
+
+- marked ``final``
+- of type ``RealmResults<T>`` where ``T`` is the type at the opposite
+  end of the relationship

source/includes/api-details/javascript/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,28 @@
+To define an inverse relationship, define a ``linkingObjects`` property in your
+object model. ``linkingObjects`` specifies the object type and
+property name of the relationship that it inverts.
+
+.. literalinclude:: /examples/generated/node/v12/relationships.test.snippet.define-inverse.js
+   :language: javascript
+
+**Dynamically Obtain an Inversely Linked Object**
+
+You can dynamically retrieve an object with an inverse relationship without
+defining a ``linkingObjects`` type in its schema. Remove the
+``linkingObjects`` type from your schema, so your schemas look like a standard
+**to-many** relationship. When you need to retrieve the linked object, call the
+:js-sdk:`Realm.Object.linkingObjects() <classes/Object.html#linkingObjects>`
+query.
+
+In the following continuation from the inverse relationship example, we 
+have removed the ``manufacturer`` field with type 'linkingObjects' from 
+the ``Car`` schema. An application developer creates several manufacturers 
+and car objects, and the application pushes the newly-created cars into a 
+manufacturer's ``cars`` field. 
+   
+To find the manufacturer who makes a specific car object, call ``.linkingObjects()``
+and pass the "Manufacturer" class name and "cars" field as parameters.
+
+The ``.linkingObjects()`` method returns a Results collection of objects whose
+property inverts the relationship. In this example, only one manufacturer makes
+the Sentra car model, so we can expect that manufacturer to be named Nissan.

source/includes/api-details/kotlin/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,13 @@
+To define an inverse relationship between objects, first define a 
+collection property in the parent object whose type is a 
+``RealmList<E>``, ``RealmSet<E>``, or ``RealmDictionary<E>``, where 
+``<E>`` is a ``RealmObject`` object type defined in your data model. 
+This can be a different Realm object type or the same Realm object 
+type:
+
+.. literalinclude:: /examples/generated/kotlin/Schema.snippet.define-inverse-property-parent.kt
+  :language: kotlin
+
+Then, define an immutable :kotlin-sdk:`backlinks 
+<io.realm.kotlin.ext/backlinks.html>` property in the child object of
+``RealmResults<E>``, where ``<E>`` is the parent object type:

source/includes/api-details/objectivec/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,7 @@
+To define an inverse relationship, use
+:objc-sdk:`RLMLinkingObjects
+<Classes.html#/c:objc(cs)RLMLinkingObjects>` in your object model.
+Override :objc-sdk:`+[RLMObject linkingObjectProperties]
+<Classes/RLMObject.html#/c:objc(cs)RLMObject(cm)linkingObjectsProperties>`
+method in your class to specify the object type and property name
+of the relationship that it inverts.

source/includes/api-details/swift/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,4 @@
+To define an inverse relationship, use :swift-sdk:`LinkingObjects
+<Structs/LinkingObjects.html>` in your object model. The
+``LinkingObjects`` definition specifies the object type and
+property name of the relationship that it inverts.

source/includes/api-details/typescript/model-data/relationships-inverse-relationship-description.rst

@@ -0,0 +1,28 @@
+To define an inverse relationship, define a ``linkingObjects`` property in your
+object model. ``linkingObjects`` specifies the object type and
+property name of the relationship that it inverts.
+
+.. literalinclude:: /examples/generated/node/v12/relationships.test.snippet.define-inverse.ts
+   :language: typescript
+
+**Dynamically Obtain an Inversely Linked Object**
+
+You can dynamically retrieve an object with an inverse relationship without
+defining a ``linkingObjects`` type in its schema. Remove the
+``linkingObjects`` type from your schema, so your schemas look like a standard
+**to-many** relationship. When you need to retrieve the linked object, call the
+:js-sdk:`Realm.Object.linkingObjects() <classes/Object.html#linkingObjects>`
+query.
+
+In the following continuation from the inverse relationship example, we 
+have removed the ``manufacturer`` field with type 'linkingObjects' from 
+the ``Car`` schema. An application developer creates several manufacturers 
+and car objects, and the application pushes the newly-created cars into a 
+manufacturer's ``cars`` field. 
+   
+To find the manufacturer who makes a specific car object, call ``.linkingObjects()``
+and pass the "Manufacturer" class name and "cars" field as parameters.
+
+The ``.linkingObjects()`` method returns a Results collection of objects whose
+property inverts the relationship. In this example, only one manufacturer makes
+the Sentra car model, so we can expect that manufacturer to be named Nissan.