Add documentation for custom model actions

- Add plugin development guide for registering custom actions
- Update admin permissions docs to mention custom actions UI
- Add docstrings to ModelAction and register_model_actions

docs/plugins/development/permissions.md

@@ -0,0 +1,36 @@
+# Custom Model Actions
+
+Plugins can register custom permission actions for their models. These actions appear as checkboxes in the ObjectPermission form, making it easy for administrators to grant or restrict access to plugin-specific functionality without manually entering action names.
+
+For example, a plugin might define a "sync" action for a model that syncs data from an external source, or a "bypass" action that allows users to bypass certain restrictions.
+
+## Registering Model Actions
+
+To register custom actions for a model, call `register_model_actions()` in your plugin's `ready()` method:
+
+```python
+# __init__.py
+from netbox.plugins import PluginConfig
+
+class MyPluginConfig(PluginConfig):
+    name = 'my_plugin'
+    # ...
+
+    def ready(self):
+        super().ready()
+        from utilities.permissions import ModelAction, register_model_actions
+        from .models import MyModel
+
+        register_model_actions(MyModel, [
+            ModelAction('sync', help_text='Synchronize data from external source'),
+            ModelAction('export', help_text='Export data to external system'),
+        ])
+
+config = MyPluginConfig
+```
+
+Once registered, these actions will appear grouped under your model's name when creating or editing an ObjectPermission that includes your model as an object type.
+
+::: utilities.permissions.ModelAction
+
+::: utilities.permissions.register_model_actions