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/docs.json

@@ -52,6 +52,10 @@
     ]
   },
   "redirects": [
+    {
+      "source": "/install/migrating-matrix",
+      "destination": "/channels/matrix-migration"
+    },
     {
       "source": "/help/gpt54-codex-agentic-parity",
       "destination": "/help/gpt55-codex-agentic-parity"
@@ -984,8 +988,7 @@
                     "pages": [
                       "install/migrating",
                       "install/migrating-claude",
-                      "install/migrating-hermes",
-                      "install/migrating-matrix"
+                      "install/migrating-hermes"
                     ]
                   },
                   "install/uninstall",
@@ -1050,6 +1053,7 @@
                   "channels/imessage",
                   "channels/bluebubbles",
                   "channels/matrix",
+                  "channels/matrix-migration",
                   "channels/matrix-push-rules"
                 ]
               },