docs: add sample code for usage ResponseTrait in

user_guide_src/source/outgoing/api_responses/018.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Libraries;
+
+use CodeIgniter\API\ResponseTrait;
+use CodeIgniter\HTTP\Response;
+
+class MyCustomClass
+{
+    use ResponseTrait;
+
+    protected Response $response;
+
+    public function __construct()
+    {
+        // Manually create a Response object
+        $this->response = service('response');
+    }
+
+    public function createUser()
+    {
+        return $this->respondCreated(['message' => 'User created.']);
+    }
+}