Add docs for rattail backup command

docs/backup/features/dbdump.rst

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+
+.. highlight:: ini
+
+================
+ Database Dumps
+================
+
+Rattail can dump all of your databases to file so that they can be
+included in the filesystem backup that happens via ``rsync`` and/or
+``borg create``.
+
+It only supports PostgreSQL and MySQL/MariaDB but those should work
+out of the box.  It will figure out which databases are present, then
+dump each to its own file.  Note that it puts them all in the
+``/root/data`` folder by default.
+
+
+Configuration
+=============
+
+You can enable the database dump feature like so::
+
+   [rattail.backup]
+   dbdump = true
+
+Note that the dbdump feature is *enabled* by default; you can disable
+like so::
+
+   [rattail.backup]
+   dbdump = false
+
+If you don't like ``/root/data`` then you can change output folder::
+
+   [rattail.backup]
+   dbdump.output = /my/preferred/folder
+
+Exclude certain databases which you do not need backed up (note, it
+does not matter if these live in PostgreSQL or MySQL, Rattail will
+just ignore these regardless where they're found)::
+
+   [rattail.backup]
+   dbdump.exclude = mytestdb, anotherdb
+
+Alternate syntax for that, useful for longer lists::
+
+   [rattail.backup]
+   dbdump.exclude = 
+       mytestdb
+       anotherdb
+
+Or, *only* include certain databases and exclude all others::
+
+   [rattail.backup]
+   dbdump.include = 
+      importantdb
+      anotherdb
+
+If you want, Rattail can dump *all* tables individually as well as
+dumping the databases themselves.  Each database file will be
+accompanied by a folder which will contain the table files::
+
+   [rattail.backup]
+   dbdump.dump_tables = true