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.

Beginne mit content/docs/code-splitting.md und sieh dir anschließend das Szenario des Issues zu serverseitigem React und einer benutzerdefinierten webpack-Konfiguration an. Entscheide, ob die Dokumentation die Auswahl benutzerdefinierter Server-/Client-Bundles abdecken oder react-loadable demonstrieren sollte. Erledigt ist die Aufgabe, wenn die Dokumentation das serverseitige Code-Splitting für Apps, die weder Next noch Gatsby verwenden, klar erklärt.