Specify additional tested autopilot boards

[ES-Alexander]

Pixhawk1 is past EOL and we don't yet have a supported alternative.

Should add additional boards to the "Recommended Autopilots" section in introduction/hardware-options/required-hardware/autopilot.md.

Happy to make a PR but don't want to add boards without a product link.

I'm assuming the Pixhawk 4 we discussed was the one from Holybro (which is listed in the ArduPilot common-autopilots page). They don't seem to mention a Pixhawk 2 anywhere on that page, so I searched for it and it's apparently also called the Cube, of which there are 5 options on the common autopilots page - is there a particular one I should link to?

[ES-Alexander]

@jaxxzer has tested Pixhawk 2 (Cube Black) and Pixhawk 4 and confirmed they work. They may require additional setup though, so waiting with this until it's confirmed that they can and do work as expected when running ArduSub.

[ES-Alexander]

Pixhawk 4 requires ArduSub >= 4.2 to enable Bar30 and leak sensor, as per here.