docs(matrix): move migration guide from install/ to channels/

The Matrix migration guide is plugin-upgrade content (encrypted-state recovery,
device verification, room-key restore) rather than a cross-system import or
machine move, so it belongs alongside the Matrix channel docs rather than under
Install > Maintenance > Migrating.

- Move docs/install/migrating-matrix.md to docs/channels/matrix-migration.md
- Update inbound link in docs/channels/matrix.md
- Update the migrating.md hub: replace the Matrix Card with a one-line link in 'Upgrade a plugin in place'
- Refresh Related list on the moved page (link Matrix push rules and Migration guide hub)
- docs.json: remove install/migrating-matrix from Maintenance > Migrating, slot channels/matrix-migration between channels/matrix and channels/matrix-push-rules in the Mainstream channels group, and add a /install/migrating-matrix -> /channels/matrix-migration redirect

docs/install/migrating.md

@@ -117,13 +117,9 @@ On the new machine, confirm:
 
 ## Upgrade a plugin in place
 
-In-place plugin upgrades preserve the same plugin id and config keys but may move on-disk state into the current layout. The Matrix plugin is the largest example because of its encrypted-state recovery requirements.
+In-place plugin upgrades preserve the same plugin id and config keys but may move on-disk state into the current layout. Plugin-specific upgrade guides live alongside their channels:
 
-<CardGroup cols={1}>
-  <Card title="Matrix plugin migration" href="/install/migrating-matrix" icon="key">
-    Encrypted-state recovery limits, automatic snapshot behavior, and manual recovery commands for the Matrix plugin.
-  </Card>
-</CardGroup>
+- [Matrix migration](/channels/matrix-migration): encrypted-state recovery limits, automatic snapshot behavior, and manual recovery commands.
 
 ## Related