Merge branch 'docs-how-backups-work' into 'master'

Add document describing how backups work

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138129



Merged-by: Evan Read <eread@gitlab.com>
Reviewed-by: Evan Read <eread@gitlab.com>
Co-authored-by: James Fargher <jfargher@gitlab.com>