docs: add howto combine migrations

docs/contributing/howto/combine-migrations.md

@@ -0,0 +1,44 @@
+---
+tags:
+  - Django
+  - migrations
+---
+
+# Combine migrations
+
+This howto assumes we have `django-linear-migrations` installed, and that we have a migration file `max_migration.txt` for each app that contains the latest migration for that app.
+
+To combine migrations `0025-0027` in the `core` app, where the last migration is `0027`
+
+1. Roll back migrations to before all the target migrations, e.g. `0024`
+
+    ```bash
+    ./script/migrate.sh core 0024
+    ```
+
+1. Delete migration files to be combined.
+
+    ```bash
+    rm app/core/migrations/00{25,26,27}_*
+    ```
+
+1. Recreate `max_migration.txt`
+
+    ```bash
+    docker-compose exec web python manage.py create_max_migration_files --recreate core
+    # the file core/max_migration.txt should now contain "0024_..."
+    ```
+
+1. Generate combined migration file
+
+    ```bash
+    ./script/migrate.sh core
+    ```
+
+## When to use this
+
+- PRs can have multiple migration files that should be made into a single create migration for merging.
+
+## Who should use this
+
+- Developers

mkdocs.yml

@@ -30,6 +30,8 @@ plugins:
         - Onboarding
         - database
         - postgres
+        - Django
+        - migrations
 
 markdown_extensions:
   - abbr