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.

Comienza con content/docs/code-splitting.md y luego revisa el escenario del issue sobre React del lado del servidor y una configuración personalizada de webpack. Decide si la documentación debe cubrir la selección personalizada de bundles de servidor/cliente o demostrar react-loadable. Se considera terminado cuando la documentación explica claramente la división de código del lado del servidor para aplicaciones que no usan Next ni Gatsby.