Add documentation on how server-side backups work

The docs for how backups work (server-side or normal) are scattered around various pages. E.g. https://docs.gitlab.com/ee/architecture/blueprints/repository_backups/ and https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/gitaly-backup.md

We should write a customer facing page about how backups work (not just server-side).

Specifically I think it would be useful to point out things like that repository backups always need the gitlab-rails DB, where data is stored/ transmitted and praefect concerns.