Docs: document --backupdb argument for inv docker.manage (#12617)

Closes #7761

## Summary

- Documents the `--backupdb` argument for `inv docker.manage`
- Describes the backup filename pattern:
`dump_<DD-MM-YYYY>_<HH_MM_SS>__<git-hash>.sql`
- Adds restore instructions with step-by-step commands

Note: #8708 proposes a `restoredb` invoke task to automate restoration.
The manual workflow documented here can coexist with that automation if
it lands.

## Test plan

- [x] Built docs locally with `make PROJECT=dev html` - renders
correctly
- [x] Tested `inv docker.manage --backupdb showmigrations` - backup file
created with documented pattern

Generated with [Claude Code](https://claude.com/claude-code)
Steered and reviewed by @nikblanchet (mostly human)

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: nikblanchet

docs/dev/install.rst

@@ -150,10 +150,43 @@ save some work while typing docker compose commands. This section explains these
 ``inv docker.manage {command}``
     Executes a Django management command in a container.
 
+    * ``--backupdb`` creates a database backup before running the command.
+      The backup is saved to the current directory with a timestamped filename:
+      ``dump_<DD-MM-YYYY>_<HH_MM_SS>__<git-hash>.sql``
+
     .. tip::
 
        Useful when modifying models to run ``makemigrations``.
 
+    .. tip::
+
+       Use ``--backupdb`` when running ``migrate`` to create a backup before applying migrations:
+
+       .. code-block:: bash
+
+          inv docker.manage --backupdb migrate
+
+       To restore from this backup, follow these steps:
+
+       .. code-block:: bash
+
+          # Start only the database container
+          inv docker.compose 'start database'
+
+          # Copy the backup file into the container (replace with your actual filename)
+          docker cp dump_24-11-2025_10_30_00__abc1234.sql community_database_1:/tmp/dump.sql
+
+          # Open a shell in the database container
+          inv docker.shell --container database
+
+       Then inside the database container:
+
+       .. code-block:: bash
+
+          dropdb -U docs_user docs_db
+          createdb -U docs_user docs_db
+          psql -U docs_user docs_db < /tmp/dump.sql
+
 ``inv docker.down``
     Stops and removes all containers running.