React in web-component tutorial missing unmount

We are using a react component through a web-component inspired by the snippet in docs.

We stumbled over an issue after which we noticed that the component is never unmounted.

We added a disconnectedCallback like this

disconnectedCallback() {
    ReactDOM.unmountComponentAtNode(this)
  }

I could make a quick PR adding that if you folks think that makes sense!

从 issue 中链接的 web-components 教程开始，尤其关注展示在自定义元素中使用 React 的示例。使用 issue 中描述的 callback 和 unmount API，更新示例以说明元素断开连接时的清理。完成的标准是教程清楚地涵盖卸载过程，并且示例仍然有效。