Companion Docs need update

[hamishwillee]

At the moment we have a scattering of docs explaining how to set up companion computers and how to install key software. But nothing that covers it end to end.

Propose we define a small set of "supported" companion computers under http://dev.px4.io/master/en/companion_computer/pixhawk_companion.html and explain how to work with them on a single OS - e.g. Ubuntu 18.04

We could cover: how to connect for development, how to connect to Pixhawk, how to flash image, how to set up wifi, how to install ROS/SDK, how to set up so can be powered from battery when used on drone.

What we're after is docs that cover the practicalities on the companions that are used most. But we also want to be able to maintain these docs.

[FYI, case that started this is https://github.com//pull/1000 - the doc http://dev.px4.io/master/en/qgc/video_streaming.html uses wayback links for Odroid setup. We should have supported boards and verify this stuff on those. ]