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 I am a bit worried that the version numbers will get outdated pretty soon.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
@jpihl What did you mean by this sentence?
"But I also like the other address better, using that we can change the port without having to change all our documentation."
What should be the standard buildbot URL in your opinion?