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.

Commencez par content/docs/code-splitting.md, puis examinez le scénario de l’issue concernant React côté serveur et une configuration webpack personnalisée. Déterminez si la documentation doit couvrir la sélection personnalisée des bundles serveur/client ou présenter react-loadable. Le travail est terminé lorsque la documentation explique clairement la séparation du code côté serveur pour les applications qui n’utilisent ni Next ni Gatsby.