Add docs for rattail backup command

docs/backup/features/index.rst

@@ -0,0 +1,22 @@
+
+==========
+ Features
+==========
+
+As mentioned earlier, Rattail's backup logic can do the following:
+
+* dump databases to file
+* run ``rsync`` to copy files somewhere
+* run ``borg create`` to make Borg archive(s) somewhere
+
+Note that the database dump always happens first, so that the
+resulting files can be included in the ``rsync`` or ``borg create``
+runs (if applicable).
+
+.. toctree::
+   :maxdepth: 3
+   :caption: Contents:
+
+   dbdump
+   rsync
+   borg