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 I am a bit worried that the version numbers will get outdated pretty soon.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
We could write something like:

In Steinwurf we use a concept called mkspec, short for makespec, this is used for informing the build system how to build to code.

And then I would just use the work mkspec as that is the one we use almost everywhere else i believe.