docs(api): update webhook configuration and add notification docs

- Simplify webhook headers and body configuration format
- Add documentation for available notification methods and parameters

docs/api.md

@@ -1619,12 +1619,8 @@ key=ListenAddr
     "Notification.Method": "webhook",
     "Webhook.Method": "POST",
     "Webhook.URL": "https://example.com/webhook",
-    "Webhook.Headers": {
-      "Content-Type": "application/json"
-    },
-    "Webhook.Body": {
-      "text": "Super-frpc notification"
-    }
+    "Webhook.Headers": "Content-Type: application/json",
+    "Webhook.Body": "{\"subject\":\"Super-frpc Alert - {{ exceptionType }}\",\"text\":\"Super-frpc Alert\",\"html\":\"{{ serviceName }} - {{ exceptionMsg }}\"}"
   }
 }
 ```
@@ -1769,4 +1765,21 @@ The API implements rate limiting to prevent abuse:
 - Login attempts: Maximum 5 attempts per minute per IP
 - All other endpoints: 100 requests per minute per token
 
-Exceeding rate limits will result in temporary IP or token blocking.
+Exceeding rate limits will result in temporary IP or token blocking.
+
+---  
+
+## Notification
+- Available notification methods
+
+| Method | Description |
+| :---: | :---: |
+| `webhook` | Webhook notification |
+
+- Replacable parameters
+
+| Parameter | Description |
+| :---: | :---: |
+| `{{ exceptionType }}` | Type of exception that triggered the notification |
+| `{{ serviceName }}` | Name of the service that experienced the exception |
+| `{{ exceptionMsg }}` | Detailed exception message |