Better intro and API docs for common tasks

[matthewryanscott]

The docs don't make it clear how to do common things, such as writing a Project or Synth to a file using write_to, or playing back projects using metrasynth/sunvox-dll-python.

The API reference at https://radiant-voices.readthedocs.io/en/latest/api-reference/index.html also does not include information about classes like Project and Synth.