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 I am a bit worried that the version numbers will get outdated pretty soon.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
@petya2164 

I like buildbot.steinwurf.dk/.com better than buildbot.steinwurf.dk/com:12344, as Morten says - for some people it looks weird.

I don't care whether we use .com or .dk.