docs: add guidance about optional services in migration guide

Explicitly mention checking config.php for optional services like Redis,
Memcached, and other dependencies before migration. Addresses issue where
migrations fail due to missing Redis configuration.

Signed-off-by: skjnldsv <skjnldsv@protonmail.com>

Author: skjnldsv

admin_manual/maintenance/migrating.rst

@@ -37,6 +37,20 @@ the new location. It is also assumed that the authentication method
     your original machine was installed recently just copying that base
     configuration is a safe best practice.
 
+    .. important:: Before beginning the migration, **check the ``config/config.php`` file
+       on your ORIGINAL system** to identify any optional services that are configured,
+       such as:
+
+       * Redis or Memcached (for caching/sessions)
+       * External object storage (S3, etc.)
+       * LDAP
+       * Mail server settings
+       * Full-text search backends (Elasticsearch, etc.)
+
+       **You must also install and configure these same services on the NEW machine
+       before copying the Nextcloud files.** Failure to do so may cause errors such
+       as "Redis server went away" or connection failures during Nextcloud startup.
+
 
 #.  On the original machine then stop Nextcloud. First activate the
     maintenance mode. After waiting for 6-7 minutes for all sync clients to