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.

Start with content/docs/code-splitting.md, then review the issue's server-side React and custom webpack configuration scenario. Decide whether the documentation should cover custom server/client bundle selection or demonstrate react-loadable. Done means the docs clearly explain server-side code splitting for apps not using Next or Gatsby.