Add BetaFPV Beta75X 2S Brushless Whoop Docs

[hamishwillee]

Add docs for BetaFPV Beta75X 2S Brushless Whoop

• Along the lines of https://docs.px4.io/en/flight_controller/bebop.html ? i.e.
  • how to build for it,
  • How to connect to it
  • how to upload new software/new bootloaders
  • How do debug it
  • Any special config
  • How to report bugs for it

Discussion originates in https://github.com/PX4/px4_user_guide/pull/411

[hamishwillee]

What do you think of the idea of putting this along with aero, crazyflie, bebop under a "Commercial Platforms", "Complete Vehicles", or similar parent folder - would be lead by you on name. Ie they all share the fact that they are not boards but full vehicles, albeit some are reference platforms.

[bkueng]

Yes I think something like "Complete Vehicles" makes sense - full/ready-to-fly vehicles where you can flash upstream PX4. On that note we should also add the Teal One (@dakejahl).