Problem on "REST API Quick start" — missed content negotiator

### Description

It should be mentioned about `ContentNegotiatorDataResponseMiddleware` in "Returning JSON responses" section.

Usage example: https://github.com/yiisoft/app-api/blob/master/config/web/di/application.php

Link to page: [Quick start](https://yiisoft.github.io/docs/guide/rest/quick-start.html)