(DOCSP-44020): Documented backup between S3 stores. (#803)

@ejafarli, I added content for the UI steps. Where can I find more
information about the new public endpoint?

<!-- Add a description of your PR here (optional) -->

- [DOCSP-44020](https://jira.mongodb.org/browse/DOCSP-44020)
- [STAGING:
Migrate](https://deploy-preview-803--docs-ops-manager.netlify.app/admin/backup/jobs-page/#migrate-one-s3-compatible-snapshot-store)
- [STAGING:
API](https://deploy-preview-803--docs-ops-manager.netlify.app/reference/api/backup/update-backup-config/)

### Self-Review Checklist

- [ ]
[Define](https://wiki.corp.mongodb.com/display/DE/Taxonomy+tagging+instructions)
taxonomy
[values](https://wiki.corp.mongodb.com/display/DE/Docs+Taxonomy) at top
of page.
- [ ] Add genre facets (tutorial or reference), as in this [example
PR](10gen/cloud-docs#5042).
- [ ] Add programmingLanguage (if necessary).
- [ ] Add meta keywords (if necessary).
- [ ] Resolve any new warnings or errors in the build.
- [ ] Proofread for spelling and grammatical errors.
- [ ] Check staging for rendering issues.
- [ ] Confirm links are working.

Author: corryroot

source/admin/backup/jobs-page.txt

@@ -164,3 +164,21 @@ procedural information, see
 
    * - :ref:`Configure a project to use KMIP <configure-group-kmip>`.
      - Specify the |kmip| fields and click :guilabel:`Save Changes`.
+
+.. _transition-s3:
+
+Transition an S3-Compatible Snapshot Store
+------------------------------------------
+
+From the :guilabel:`Jobs` page, you can transition between 
+S3-compatible snapshot stores without terminating the backup.
+
+After you transition to another S3-compatible snapshot store, the 
+existing snapshots remain in the previous store until they expire and 
+the :ref:`groom job <grooms-page>` removes them. All new snapshots are 
+stored on the newly selected store, starting with a full snapshot. If a 
+snapshot is in progress at the time of migration, it will complete on 
+the current store, and the migration will take effect with the next 
+scheduled snapshot.
+
+.. include:: /includes/steps/admin-transition-s3-blockstore.rst

source/includes/steps-admin-transition-s3-blockstore.yaml

@@ -0,0 +1,23 @@
+stepnum: 1
+source:
+  file: steps-source-admin-navigation.yaml
+  ref:  nav-to-jobs-page
+---
+title: "Click the link for the backup job."
+stepnum: 3
+level: 4
+ref: backup-job-link
+---
+title: "Select the Blockstore ID to transition to."
+stepnum: 3
+level: 4
+ref: blockstore-id-transition
+content: |
+
+  a. Click the :guilabel:`Blockstore ID` link.
+
+  #. Select the Blockstore ID to transition to.
+
+  #. Click :guilabel:`Update`.
+  
+...