Merge branch 'docs-backup-utility-repositories-server-side' into 'master'

Document repositories server-side backup with backup-utility

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/153358



Merged-by: Achilleas Pipinellis <axil@gitlab.com>
Co-authored-by: Michael Kozono <mkozono@gitlab.com>