(DOCSP-45304) adds api documentation for taking an on demand snapshot-v8.0-backport (633) (#755)

# Backport

This will backport the following commits from `master` to `v8.0`:
- [(DOCSP-45304) adds api documentation for taking an on demand
snapshot](10gen/docs-ops-manager#633)

<!--- Backport version: 9.6.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

Co-authored-by: Carrie Kim <[email protected]>

Author: davidhou17

source/reference/api/snapshots/take-an-on-demand-snapshot.txt

@@ -0,0 +1,124 @@
+==========================
+Take an On-Demand Snapshot
+==========================
+
+.. default-domain:: mongodb
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 2
+   :class: singlecol
+
+.. include:: /includes/api/base-api-uri.rst
+
+Resource
+--------
+
+.. code-block:: http
+
+   POST /groups/{groupId}/clusters/{clusterId}/snapshots/onDemandSnapshot
+
+Required Permissions
+--------------------
+
+You must have the :authrole:`Global Backup Admin` to 
+take an on-demand snapshot.
+
+Request Parameters
+------------------
+
+Request Path Parameters
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. list-table::
+   :widths: 10 10 80
+   :header-rows: 1
+   :stub-columns: 1
+
+   * - Name
+     - Type
+     - Description
+
+   * - GROUP-ID
+     - string
+     - Unique identifier of your project from your project settings.
+
+   * - CLUSTER-ID
+     - string
+     - Unique identifier of the cluster that the :manual:`snapshot </reference/glossary/#std-term-snapshot>` represents.
+
+Request Query Parameters
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following query parameters are optional:
+
+.. list-table::
+   :widths: 20 14 11 45 10
+   :header-rows: 1
+   :stub-columns: 1
+
+   * - retentionDays
+     - number
+     - Required
+     - Integer that indicates the number of days the the on-demand 
+       snapshot will be retained. Must be greater than 0.
+     - ``15``
+
+   * - envelope
+     - boolean
+     - Optional
+     - Flag that indicates whether or not to wrap the response in an
+       envelope.
+
+       Some |api| clients cannot access the |http| response headers or
+       status code. To remediate this, set ``envelope : true`` in the
+       query.
+
+       For endpoints that return a list of results, the ``results``
+       object is an envelope. |mms| adds the ``status`` field to the
+       response body.
+
+     - ``false``
+
+Request Body Parameters
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. include:: /includes/api/no-body-parameters.rst
+
+Response
+--------
+
+Example Request
+---------------
+
+.. code-block:: sh
+
+   curl --user "{PUBLIC-KEY}:{PRIVATE-KEY}" --digest \
+    --include \
+    --request POST "https://{opsManagerHost}:{port}/api/public/v1.0/groups/{groupId}/clusters/{clusterId}/snapshots/onDemandSnapshot"
+
+Example Response
+----------------
+
+Response Header
++++++++++++++++
+
+.. include:: /includes/api/api-headers/401.rst
+
+.. include:: /includes/api/api-headers/200.rst
+
+Response Body
++++++++++++++
+
+.. code-block:: json
+
+   {
+      "clusterName": "myCluster",
+      "description": "On demand snapshot request complete .",
+      "timestamp": {
+          "increment": 1,
+          "time": 1719518976,
+          "date": "2024-06-27T20:09:36Z"
+      }
+   }