chore: Add documentation for API channel (#1179)

docs/channels/api-channel/callback-url.md

@@ -0,0 +1,45 @@
+---
+path: '/docs/channels/api/callback-url'
+title: 'Callback URL and message payload'
+---
+
+When a new message is created in the API channel, you will get a POST request to the Callback URL specified while creating the API channel. The payload would look like this.
+
+**Event type**: `message_created`
+
+```json
+{
+  "id": 0,
+  "content": "This is a incoming message from API Channel",
+  "created_at": "2020-08-30T15:43:04.000Z",
+  "message_type": "incoming",
+  "content_type": null,
+  "content_attributes": {},
+  "source_id": null,
+  "sender": {
+    "id": 0,
+    "name": "contact-name",
+    "avatar": "",
+    "type": "contact"
+  },
+  "inbox": {
+    "id": 0,
+    "name": "API Channel"
+  },
+  "conversation": {
+    "additional_attributes": null,
+    "channel": "Channel::Api",
+    "id": 0,
+    "inbox_id": 0,
+    "status": "open",
+    "agent_last_seen_at": 0,
+    "user_last_seen_at": 0,
+    "timestamp": 0
+  },
+  "account": {
+    "id": 1,
+    "name": "API testing"
+  },
+  "event": "message_created"
+}
+```