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.

まず content/docs/code-splitting.md から始め、サーバーサイド React とカスタム webpack 設定に関する issue のシナリオを確認します。ドキュメントでカスタムのサーバー/クライアントバンドル選択を扱うべきか、react-loadable を実例として示すべきかを判断します。Next や Gatsby を使用していないアプリでのサーバーサイドコード分割について、ドキュメントが明確に説明できていれば完了です。