document cross-compile and install/sync for newer contributors

None of this is documented, and so it's hard for new contributors to do things.

To cross-compile, you need to run the docker container at https://github.com/robotpy/mostrobotpy/blob/main/.github/workflows/dist.yml#L217 and perform all the steps that are listed there.

An easier approach for cross-compilation is you make a PR against mostrobotpy, it will publish all the wheels to the CI job, and you can download the wheels from that (see for example https://github.com/robotpy/mostrobotpy/actions/runs/14223454151, the 'cross-roborio' download).