Geo: Document how to reconcile differences with Geo checksum mismatches

Right now if the primary and secondary have a checksum verification failure, it's not obvious what exactly is the problem.

The way I do this now:

1. Get the hashed path (e.g. /var/opt/gitlab/git-data/repositories/@hashed/9a/e2/9ae2bdd7beedc2e766c6b76585530e16925115707dc7a06ab5ee4aa2776b2c7b.git) from the /admin page.
2. On both the primary and secondary, run: git show-ref --head | grep -E "HEAD|(refs/(heads|tags|keep-around|merge-requests|environments|notes)/)". That last part is from https://gitlab.com/gitlab-org/gitaly/blob/be97ae4d2760e1065cbcd1fb66f37100b45699f0/internal/service/repository/calculate_checksum.go#L23.
3. Do a diff between the contents.

In the future, it would be nice if Geo did this diff automatically and showed it in the admin screen.