help: Document `unarchive_stream` management command.

- Adds "Unarchiving archived streams" section to "Archive a stream".
- Adds `./manage.py unarchive_stream` to contributor docs.

docs/production/management-commands.md

@@ -125,6 +125,9 @@ There are dozens of useful management commands under
 - `./manage.py export_single_user`: does a limited version of the [main
   export tools](export-and-import.md) containing just
   the messages accessible by a single user.
+- `./manage.py unarchive_stream`:
+  [Reactivates](https://zulip.com/help/archive-a-stream#unarchiving-archived-streams)
+  an archived stream.
 - `./manage.py reactivate_realm`: Reactivates a realm.
 - `./manage.py deactivate_user`: Deactivates a user. This can be done
   more easily in Zulip's organization administrator UI.

help/archive-a-stream.md

@@ -41,6 +41,17 @@ than archiving them.
 
     Archiving a stream is currently irreversible via the UI.
 
+## Unarchiving archived streams
+
+If you are self-hosting, you can unarchive an archived stream using the
+`unarchive_stream` [management command][management-command]. This will restore
+it as a private stream with shared history, and subscribe all organization
+owners to it. If you are using Zulip Cloud, you can [contact us](/help/contact-support)
+for help.
+
+[management-command]:
+https://zulip.readthedocs.io/en/latest/production/management-commands.html#other-useful-manage-py-commands
+
 ## Related articles
 
 * [Edit a message](/help/edit-a-message)
@@ -48,3 +59,4 @@ than archiving them.
 * [Delete a topic](/help/delete-a-topic)
 * [Message retention policy](/help/message-retention-policy)
 * [Stream permissions](/help/stream-permissions)
+* [Zulip Cloud or self-hosting?](/help/zulip-cloud-or-self-hosting)