Autotune docs

[hamishwillee]

This is first draft of autotune docs. There are still a few questions and I am hoping for a better video and to understand how to test this on gazebo.

It is very similar to the AMC guide except you can skip a lot of it you want and it focusses more on the default than the options. So basically it talks about the normal setup and you don't have to know about configuration.
It also is less "you need an experienced engineer" and more "this can fail, monitor and test"

Does it "work" for you?
If so then QGC would be a very much cut down version.

[hamishwillee]

@bresch That was helpful thanks. I have updated and restructured. Now happy :-).

The essential difference:

• The stuff that was "overview" is now Background/Detail. The introduction has enough so that you know the risks and what is covered. I like this because now you jump straight in. It also means that docs for QGC would pretty much be the same as for PX4 but without everything below the heading "Developer SDKs".
• The process just includes the recommended default steps. The fact that you can tune in position mode for example is an "Additional note".

[hamishwillee]

@bresch Thanks for your help. Did some minor tidy and merged this. Might add a video from mavlink/qgroundcontrol#9904 (comment) later - asking @mrpollo to get it on the PX4 youtube channel.

For the manual tuning docs, what's the message? I propose we just add a header saying that autotuning is a much easier way of doing all of this. OK?