Document robot program startup

[virtuald]

This is the contents of robotCommand (roborio, but systemcore will be similar):
    https://github.com/robotpy/robotpy-installer/blob/main/robotpy_installer/cli_deploy.py#L617

That calls `python -m robotpy` with a `run` argument:

    https://github.com/robotpy/robotpy-cli/blob/main/robotpy/main.py#L197

That function is the thing that finds `robot.py` and loads a robot class from it. It then loads the `run` entrypoint and passes the robot class to it.

That entrypoint is in the wpilib package:

    https://github.com/robotpy/mostrobotpy/blob/main/subprojects/robotpy-wpilib/pyproject.toml#L36

Which causes this class's `run` function to be called:

    https://github.com/robotpy/mostrobotpy/blob/main/subprojects/robotpy-wpilib/wpilib/_impl/start.py#L89

Which then calls the `main` function of the robot class, which then does whatever it needs to do to continue getting the robot to work.