Add plugin dev documentation for webhook callbacks

docs/plugins/development/webhooks.md

@@ -0,0 +1,63 @@
+# Webhooks
+
+NetBox supports the configuration of outbound [webhooks](../../integrations/webhooks.md) which can be triggered by custom [event rules](../../features/event-rules.md). By default, a webhook's payload will contain a serialized representation of the object, before & after snapshots (if applicable), and some metadata.
+
+## Callback Registration
+
+Plugins can register callback functions to supplement a webhook's payload with their own data. For example, it might be desirable for a plugin to attach information about the status of some objects at the time a change was made.
+
+This can be accomplished by defining a function which accepts a defined set of keyword arguments and registering it as a webhook callback. Whenever a new webhook is generated, the function will be called, and any data it returns will be attached to the webhook's payload under the `context` key.
+
+### Example
+
+```python
+from extras.webhooks import register_webhook_callback
+from my_plugin.utilities import get_foo_status
+
+@register_webhook_callback
+def set_foo_status(object_type, event_type, data, request):
+    if status := get_foo_status():
+        return {
+            'foo': status
+        }
+```
+
+The resulting webhook payload will look like the following:
+
+```json
+{
+    "event": "updated",
+    "timestamp": "2025-08-07T14:24:30.627321+00:00",
+    "object_type": "dcim.site",
+    "username": "admin",
+    "request_id": "49e3e39e-7333-4b9c-a9af-19f0dc1e7dc9",
+    "data": {
+        "id": 2,
+        "url": "/api/dcim/sites/2/",
+        ...
+    },
+    "snapshots": {...},
+    "context": {
+        "foo": 123
+    }
+}
+```
+
+### Callback Function Arguments
+
+| Name          | Type              | Description                                                       |
+|---------------|-------------------|-------------------------------------------------------------------|
+| `object_type` | ObjectType        | The ObjectType which represents the triggering object             |
+| `event_type`  | String            | The type of event which triggered the webhook (see `core.events`) |
+| `data`        | Dictionary        | The serialized representation of the object                       |
+| `request`     | NetBoxFakeRequest | A copy of the request (if any) which resulted in the change       |
+
+## Where to Define Callbacks
+
+Webhook callbacks can be defined anywhere within a plugin, but must be imported during plugin initialization. If you wish to keep them in a separate module, you can import that module under the PluginConfig's `ready()` method:
+
+```python
+def ready(self):
+    super().ready()
+    from my_plugin import webhook_callbacks
+```

mkdocs.yml

@@ -144,6 +144,7 @@ nav:
             - Search: 'plugins/development/search.md'
             - Event Types: 'plugins/development/event-types.md'
             - Data Backends: 'plugins/development/data-backends.md'
+            - Webhooks: 'plugins/development/webhooks.md'
             - REST API: 'plugins/development/rest-api.md'
             - GraphQL API: 'plugins/development/graphql-api.md'
             - Background Jobs: 'plugins/development/background-jobs.md'