Specify additional tested autopilot boards

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](https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_ChibiOS/hwdef/Pixhawk4/README.md) (which is listed in the [ArduPilot common-autopilots page](https://ardupilot.org/copter/docs/common-autopilots.html)). 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?

Read introduction/hardware-options/required-hardware/autopilot.md and its “Recommended Autopilots” section, then compare the candidate boards with the linked ArduPilot common-autopilots page. Confirm which Pixhawk or Cube products should be linked before editing. Done means the section lists supported alternatives to Pixhawk1 with product links.

Written by the indexing model from the issue text.