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!

從 issue 中連結的 web-components 教學開始，尤其關注展示在自訂元素中使用 React 的範例。使用 issue 中描述的 callback 和 unmount API，更新範例以說明元素中斷連線時的清理。完成的標準是教學清楚地涵蓋卸載流程，且範例仍然有效。