[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

docs/getting_started/index.md

@@ -15,7 +15,7 @@ GoToSocial supports both SQLite and Postgres and you can start using either. We
 SQLite is great for a single-user instance. If you're planning on hosting multiple people it's advisable to use Postgres instead. You can always use Postgres regardless of the instance size.
 
 !!! tip
-    Please backup your database. The database contains encryption keys for the instance and any user accounts. You won't be able to federate again from the same domain if you lose these keys.
+    Please [backup  your database](../admin/backup_and_restore.md). The database contains encryption keys for the instance and any user accounts. You won't be able to federate again from the same domain if you lose these keys.
 
 ## Domain name