Turn install instructions into install scripts for each platform

[Ezward]

Our installation instructions are fairly complex and it's not that hard to skip a step by accident. It would be better to have a script that installs everything with a single command. This is easier for users and will probably be easier for maintainers as well, since we can simpify the documentation. What a platform is rev'd, then we can just update the script without a need to update the documentation.

• one script per platform, starting with RaspberryPi and Jetson Nano, then Mac, Linux and Windows
• The script should install everything with a single command.
• The script should be able to reinstall over a prior installation (so script is idempotent).

[Ezward]

This this branch for a nano install script; 990-jetson-nano-install-script