[docs] Rework backups a bit (#1942)

* [docs] Rework backups a bit

This changes the existing backup documentation to:

* Push a bit harder on people to perform backups, it's not really just a
  nice to have
* Removes the language about migrating to/from GoToSocial and a
  different ActivityPub server since that's really not supported
* Adds a section about using backup software and provides an example on
  how to do this using Borgmatic

* [docs] Remove too much info in db section

* [docs] Add docs on how to backup local media

This adds documentation pointing people at the media list-local command
in order to determine what media files they need to include as part of
their backups.

Provides a Python script that people can use to transform the media
listing from the CLI into Borg patterns. It also includes a Borgmatic
config.yaml in the repository so people can easily fetch it instead of
copy-pasting from the docs.

* [bugfix] Ensure we emit an absolute path prefix

It works either way, as a pattern like data/files/<ID> would match a
file on /data/files/<ID>. But it would potentially also match any path
that happens to include data/files/<ID> but not rooted at the
storage-local-base-path.

* [docs] Add more links to media list CLI reference

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