Add crosscompile docs

@mortenvp @jpihl Take a look when you have time. I tried to be very thorough with the instructions. I am a bit worried that the version numbers will get outdated pretty soon.
RTD link: https://kodo.readthedocs.org/en/add-crosscompile-docs/cross_compile.html
By adding the port the site is more likely to be available, as the apache server is sometimes a bit slow to make the site available after a deployment.
But I also like the other address better, using that we can change the port without having to change all our documentation.