[ADD] Attempt to explain the sentinel user function

Let's see if the docs are generated properly with this

docs/development/custom/aa-framework.md

@@ -7,5 +7,6 @@ Alliance Auth is providing its own CSS framework with a couple of CSS classes.
 .. toctree::
     :maxdepth: 1
 
+    framework/api
     framework/css
 ```

docs/development/custom/framework/api.md

@@ -0,0 +1,28 @@
+# Alliance Auth Helper-Functions API
+
+## User API
+
+### get_main_character_from_user
+
+### get_main_character_name_from_user
+
+### get_sentinel_user
+
+This function is useful in models when using `User` model-objects as foreign keys.
+Django needs to know what should happen to those relations when the user is being
+deleted. To keep the data, you can have Django map this to the sentinel user.
+
+Import:
+
+```python
+from allianceauth.framework.api.user import get_sentinel_user
+```
+
+And later in your model:
+
+```python
+creator = models.ForeignKey(
+    to=User,
+    on_delete=models.SET(get_sentinel_user),
+)
+```