Add get_airspeed method to lua bindings

[AlexisMGL]

Add get_airspeed(instance) method, useful for planes with multiple sensors (currently, only the primary sensor is available).

• Added AP_Airspeed chapter in docs.lua and bindings.desc.
• Implemented airspeed:get_airspeed method.
• Tested in SITL with the following_script:

-- This script gives airspeed value if available

function state_init()
gcs:send_text(6, 'script initiated')
return state_update, 500
end

function state_update()

local message = string.format("Airspeed: %.2f m/s", airspeed:get_airspeed(0))
gcs:send_text(6,message)
return state_update, 6000

end

return state_init,500

[IamPete1]

We do have a existing PR for this. #20499

I do think you at least also need a binding for healthy.

The further problem is that if you call the healthy method with a invalid index you will read past the end the array. Is the unsafe methods that have prevented us from adding bindings so far.....

[peterbarker]

@AlexisMGL please compare/contrast to the PR from @MattKear which @IamPete1 pointed out

[AlexisMGL]

I've added other airspeed methods from #20499. It's unclear to me if I should keep the range check, for instance, or not.

[peterbarker]

@AlexisMGL this needs to pass CI...

[AlexisMGL]

I can rebase if needed