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 I am a bit worried that the version numbers will get outdated pretty soon.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
Bongo is only hosting the automatically generated toolchains (Android). We can migrate in the future, but not now.
The raspberry toolchain has some special names, so the mkspec will not work if people build their own version. It is much safer to use ours. Our crosstools guide is private and it's also complicated. I don't think we should expose that complexity to the general public ;)