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 I am a bit worried that the version numbers will get outdated pretty soon.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 would love to have a guide showing me how to do this.. because I don't know.. For android we point to our Android guide.. maybe we can have something similar for iOS. I mean running a command-line binary on iOS.