Add developer setup docs, move some README contents there

[rmunn]

We could trim down the README by moving developer setup instructions to a different document. That way the tips like "on Windows, you need to do this to allow Powershell scripts to run" or "on Linux, do this to prevent docker-desktop from filling up /var/log/syslog" could live in different files and the README could be shorter.

I suggest creating DEVELOPER-win.md, DEVELOPER-osx.md, and DEVELOPER-linux.md and linking them from the main README, then moving several of the OS-specific README sections into those files.