Update user guide

docs/user_guide.md

@@ -234,3 +234,49 @@ All the data generated by the server is located in the `data/` directory:
  - Uploaded media
 
 Restoring is as easy as adding your backed up `data/` directory into a fresh deployment.
+
+## Tasks
+
+### Pruning old data
+
+You should prune old data from time to time to free disk space.
+
+The default retention for the inbox data is 15 days.
+
+It's configurable via the `inbox_retention_days` config item in `profile.toml`:
+
+```toml
+inbox_retention_days = 30
+```
+
+Data owned by the server will never be deleted (at least for now), along with:
+
+ - bookmarked objects
+ - liked objects
+ - shared objects
+ - inbox objects mentioning the local actor
+ - objects related to local conversations (i.e. direct messages, replies) 
+
+For now, it's recommended to make a backup before running the task in case it deletes unwanted data.
+
+You should shutdown the server before running the task.
+
+#### Python edition
+
+```bash
+# shutdown supervisord
+cp -r data/microblogpub.db data/microblogpub.db.bak
+poetry run inv prune-old-data
+# relaunch supervisord and ensure it works as expected
+rm data/microblogpub.db.bak
+```
+
+#### Docker edition
+
+```bash
+docker compose stop
+cp -r data/microblogpub.db data/microblogpub.db.bak
+make prune-old-data
+docker compose up -d
+rm data/microblogpub.db.bak
+```