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.

Inizia da content/docs/code-splitting.md, quindi esamina lo scenario dell’issue relativo a React lato server e a una configurazione webpack personalizzata. Decidi se la documentazione debba trattare la selezione personalizzata dei bundle server/client oppure dimostrare react-loadable. Il lavoro è completo quando la documentazione spiega chiaramente il code splitting lato server per le app che non usano Next o Gatsby.