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 I am a bit worried that the version numbers will get outdated pretty soon.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
I just noticed that some of these things are already mentioned here:
https://kodo.readthedocs.org/en/add-crosscompile-docs/unittest.html
Maybe we should have a separate page for the tool options (after we are ready with the waf --help)