Add common flight modes service

[hamishwillee]

This creates a page outlining the default/common flight modes used across most flight stacks. I have "positioned" this by marking it under construction and linking to the google doc.

What wasn't at first obvious to me is that it is the proposed enum of baseline types and behaviour that a flight stack will need to match. The actual name does not - so if a flight stack was, for example, to call their offboard mode "Guided" that is fine if it emits or is set using the same enum.

@LorenzMeier Is the next step to invite other flight stacks to see if these definitions map sufficiently to their modes? Then define the new baseline mode enum?

First part of #196

[hamishwillee]

It is desirable for this to map to ArduPilot Circle mode. Both modes circle facing inwards, and both will automatically do so without user intervention.

Differences are that circle mode allows altitude and yaw to be changed with sticks, default rotation speed and direction is set by parameter (rather than hard coded initial value with stick override).

Can these be somehow treated as one, or are these differences too great (IMO yes, too different). These are like sub modes.

[auturgy]

As per my comment in https://docs.google.com/document/d/1LIcfOL3JrX-EznvXArna1h-sZ7va7LRTteIUzISuD8c/edit I think starting small, and picking a non-contentious subset of truly common modes, is going to aid moving this forward.

[hamishwillee]

@auturgy Of the set here, can you recommend the small subset you might consider acceptable for first discussion?

I'm more than happy with the concept, though depending on how the new definition of modes falls out, this might actually make things harder in the long run. So the largest minimal set reasonable :-)