Add documentation to backup PeerTube

[hecko-yes]

Describe the desired information/correction

I want to make regular backups of the instance (not just the videos themselves) in case of a storage failure. The migration guide comes close, but it requires the server to be shut down while exporting the database, which feels suboptimal for daily backups.

(It would also be nice to know how to exclude lower-resolution transcodes to save on storage, but this is not critical and it seems difficult to achieve perfectly with the current storage layout.)

[hecko-yes]

Helpful forum thread: https://framacolibri.org/t/sensible-backup-strategy-using-a-synology-nas/17533