Add docs for rattail backup command

docs/backup/features/borg.rst

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+
+.. highlight:: ini
+
+======
+ Borg
+======
+
+Rattail can invoke the ``borg create`` command, to make one or more
+Borg archives at configured locations.  For more about Borg, see
+`<https://www.borgbackup.org/>`_.
+
+This allows for multiple destinations so that to be extra super safe,
+you can e.g. make archives at each of:
+
+* second drive attached to local machine
+* path on remote machine on local network
+* path on remote machine in the cloud
+
+Rattail also will invoke the ``borg prune`` command to keep the
+archive repositories from growing too large etc.
+
+
+Configuration
+=============
+
+You can enable the borg feature like so::
+
+   [rattail.backup]
+   borg.enabled = true
+
+The borg feature is *disabled* by default but you can make that
+explicit::
+
+   [rattail.backup]
+   borg.enabled = false
+
+
+Destination(s)
+--------------
+
+Rattail supports multiple destinations for Borg archives; it will run
+``borg create`` (and also ``borg prune``) separately for each defined.
+
+As an example we'll show config with 3 destinations; hopefully from
+here you can figure out how to do what you need::
+
+   [rattail.backup]
+   borg.remotes = 
+      mydrive
+      otherserver
+      cloudserver
+
+   borg.remote.mydrive.repo = /mnt/mydrive/borg-storage/myserver
+   borg.remote.mydrive.passphrase = sooperseekrit
+
+   borg.remote.otherserver.repo = borguser@otherserver:/srv/borg-storage/myserver
+   borg.remote.otherserver.passphrase = sooperseekrit
+
+   borg.remote.cloudserver.repo = borguser@cloudserver:/srv/borg-storage/myserver
+   borg.remote.cloudserver.passphrase = sooperseekrit
+
+The above uses the same Borg encryption passphrase for all 3
+repositories but they could be different if you wanted.
+
+If destination is on another server, in some cases you must tell
+Rattail where the Borg binary lives on that server, e.g.::
+
+   [rattail.backup]
+   borg.remotes = cloudserver
+
+   borg.remote.cloudserver.repo = borguser@cloudserver:/srv/borg-storage/myserver
+   borg.remote.cloudserver.passphrase = sooperseekrit
+   borg.remote.cloudserver.borg = /path/to/borg
+
+
+Files to Backup
+---------------
+
+This works the same way as for rsync; see
+:ref:`rsync-include-exclude`.
+
+However the actual config option names can vary in the following way:
+
+If you prefer, or need to differentiate, you can specify the include
+and exclude options with ``borg`` prefix::
+
+   [rattail.backup]
+   borg.include = /home
+   borg.exclude = /home/ignoreme
+
+If you do specify them that way, Rattail will honor them.  But if you
+do not, it also will honor the same options with ``rsync`` prefix::
+
+   [rattail.backup]
+   rsync.include = /home
+   rsync.exclude = /home/ignoreme
+
+This is because anyone using both rsync *and* borg will likely want
+the same set of files backed up both ways.  Also because the rsync
+feature came first...