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.

从 content/docs/code-splitting.md 开始，然后查看 issue 中关于服务端 React 和自定义 webpack 配置的场景。决定文档应该介绍自定义服务器/客户端 bundle 的选择，还是演示 react-loadable。完成的标准是：文档清晰地解释不使用 Next 或 Gatsby 的应用如何进行服务端代码分割。