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 I am a bit worried that the version numbers will get outdated pretty soon.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
Yes, Apache is really crazy sometimes. This is the only address that works all the time!
Personally, I am a fan of the DK address. It gives us a nice Danish identity, which differentiates us from the million companies using the com domain ;)