add page about borg compact to free archive disk space

docs/backup/compact.rst

@@ -0,0 +1,15 @@
+
+============================
+ Freeing Archive Disk Space
+============================
+
+Periodically to free disk space used by Borg archive storage, run the
+``borg compact`` command with your repo URL:
+
+.. code-block:: sh
+
+   sudo borg compact --progress --verbose borg@cloud.example.com:/path/to/borg-repos/myserver
+
+The first time you do so, specify ``--cleanup-commits`` to ensure legacy cruft is removed.
+
+For more info see :doc:`borg:usage/compact`.

docs/backup/index.rst

@@ -24,3 +24,4 @@ existing Rattail app in charge of backups.
    running
    features/index
    restore
+   compact

docs/conf.py

@@ -39,6 +39,7 @@ extensions = [
 
 intersphinx_mapping = {
     'alembic': ('https://alembic.sqlalchemy.org/en/latest/', None),
+    'borg': ('https://borgbackup.readthedocs.io/en/stable/', None),
     'rattail': ('https://docs.wuttaproject.org/rattail/', None),
     'tailbone': ('https://docs.wuttaproject.org/tailbone/', None),
     'wuttjamaican': ('https://docs.wuttaproject.org/wuttjamaican/', None),