Sub: start the transfer process from ardusub.com

[ES-Alexander]

ArduSub.com has been gradually becoming redundant and outdated as we've split out and replicated/updated the control station software and onboard computer documentation elsewhere.

Transferring the remaining firmware-focused content here will allow us to

1. dismantle the old site
2. reduce fragmentation between the sub-related documentation sources
3. continue reducing the rift between ArduSub and the more general ArduPilot project, and
4. encourage Blue Robotics to contribute to the ArduPilot docs upkeep (instead of just doing our own thing)

This is the starting point for that.

I'm intending to make substantial additions to this in subsequent PRs, but it seems worthwhile for the initial PR to be relatively small, and focused on infrastructure, to allow for a more straightforward review process, and to make sure things are set up correctly before we go too deep.