Adds build instructions to each flight controller page

[ksschwabe]

This commit adds the build instructions for each of the Pixhawk series flight controller boards, so that the pages are consistent with the Pixhawk 2 page, which had the build instructions.

This will help the users to know what build instruction needs to be run to compile the firmware for a particular flight controller.

[hamishwillee]

I'm actually moving all of these boards out to the PX4 User guide (as soon as @LorenzMeier does his review). I wouldn't want those build instructions in the user guide. Instead we'd have one page with the list of controllers vs FMU/build target.

So this is good idea, but recommend we don't merge, or we do merge and understand that the info will be lost from here.

[ksschwabe]

Understood. I just wanted to be able to find what build target related to what physical hardware controller board. I couldn't find a table anywhere.

There is the following page in the Px4 User Guide, https://docs.px4.io/en/flight_controller/pixhawk_series.html, but that still doesn't tell you that you need to run make px4fmu-v4pro_default to build for the Pixhawk 3 Pro.

[hamishwillee]

@ksschwabe Completely fair enough - and it is great that you also took the effort to add the information :-)
I am updating the pixhawk series guide, and in that doc you could INFER the information. - but it isn't really the right place.

I'm going to merge this PR for now, and fix it properly with a table once the autopilots have moved to the user guide.: #282

[ksschwabe]

@hamishwillee: Sounds good. Thanks.