Add documentation to demonstrate code splitting on server side apps.

This is with reference to this link. Though i admit the documentation is pretty lucid, i feel a section can also be made for code splitting with server sided react apps. This is particularly tricky as correct bundles have to be utilised / picked up on both server and client side. This will be particularly useful for someone who is not using next / gatsby and is trying to use their own webpack config. Alternatively we can also show the above using react-loadable. I know the docs here are pertaining to just react, but adding this section may be helpful. If okay i'm cool with submitting a PR.

從 content/docs/code-splitting.md 開始，接著檢視 issue 中關於伺服器端 React 和自訂 webpack 設定的情境。決定文件應涵蓋自訂伺服器/用戶端 bundle 的選擇，還是示範 react-loadable。完成的標準是：文件清楚說明不使用 Next 或 Gatsby 的應用程式如何進行伺服器端程式碼分割。