Document how to migrate off Gitaly Cluster

The documentation includes instructions to migrate data into the Gitaly
Cluster, but instructions to migrate back off are missing.

This change adds instructions to move repositories back to direct Gitaly
storage.

Changelog: added
Issue: https://gitlab.com/gitlab-org/gitaly/-/issues/3797

doc/administration/gitaly/index.md

@@ -379,6 +379,18 @@ Some [known database inconsistency issues](#known-issues) exist in Gitaly Cluste
 remain on your current service for now. We can adjust the date for
 [NFS support removal](#nfs-deprecation-notice) if this applies to you.
 
+### Migrate off Gitaly Cluster
+
+If you have repositories stored on a Gitaly Cluster, but you'd like to migrate
+them back to direct Gitaly storage:
+
+1. Create and configure a new
+   [Gitaly server](configure_gitaly.md#run-gitaly-on-its-own-server).
+1. [Move the repositories](../operations/moving_repositories.md#move-repositories)
+   to the newly created storage. There are different possibilities to move them
+   by shard or by group, this gives you the opportunity to spread them over
+   multiple Gitaly servers.
+
 ## Monitor Gitaly and Gitaly Cluster
 
 You can use the available logs and [Prometheus metrics](../monitoring/prometheus/index.md) to