Common: optical flow setup instruction improvements

[rmackay9]

This includes a few changes to the optical flow setup pages based on difficulties that @andyp1per recently experienced. I don't think these changes are sufficient but they're a start:

1. move the rangefinder check section up to be just below the raw sensor value check
2. improve the sensor check section to include setting the EKF origin and recommend moving the vehicle around to confirm the vehicle icon on the map also moves
3. remove the out-of-date suggestion to change the arming checks. This is not required
4. discourage the use of FlowHold on the various pages that mention it

I've built this locally and it looks OK to me.

As I said, these updates are not complete so any other comments from AndyP are very much welcome!

[andyp1per]

Might need a comment here about the special case of 0.

[andyp1per]

So I think min->max should set the usable range and RNGFND1_GNDCLEAR should set the value on the ground (which is 0 in my case)