[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

example/borgmatic/config.yaml

@@ -0,0 +1,26 @@
+location:
+  repositories:
+    - path: ssh://<find it in your provider control panel>
+      label: <anything but typically the provider, for example borgbase>
+  patterns_from:
+  - /etc/borgmatic/gotosocial_patterns
+
+storage:
+  compression: auto,zstd
+  archive_name_format: '{hostname}-{now:%Y-%m-%d-%H%M%S}'
+  retries: 5
+  retry_wait: 30
+
+retention:
+    keep_daily: 7
+    keep_weekly: 6
+    keep_monthly: 12
+
+hooks:
+  before_backup:
+      - /usr/bin/systemctl stop gotosocial
+  after_backup:
+      - /usr/bin/systemctl start gotosocial
+  sqlite_databases:
+    - name: gotosocial
+      path: /path/to/sqlite.db