Add Altitude Cruise mode

-add new NAVIGATION_STATE_ALTITUDE_VOYAGER
-this mode does require manual control to enter
-but you can disable the manual control loss failsafe to continue
flying in case of manual control loss
-for MC: in throttle and yaw are controlled like in Altitude mode,
the tilt is controlled via integrated rate input (similar to Acro,
but with tilt limit)

Signed-off-by: Silvan Fuhrer <silvan@auterion.com>

docs/en/flight_modes_mc/index.md

@@ -21,10 +21,12 @@ Manual-Easy:
 - [Stabilized mode](../flight_modes_mc/manual_stabilized.md) — Releasing the sticks levels and maintains the vehicle horizontal posture (but not altitude or position).
   The vehicle will continue to move with momentum, and both altitude and horizontal position may be affected by wind.
   This mode is also used if "Manual mode" is selected in a ground station.
+- [Altitude Cruise mode](../flight_modes_mc/altitude_cruise.md) — Very similar to _Altitude mode_, with the difference that when the roll and pitch sticks are released the vehicle does not level out but keeps the tilt until further inputs are given.
+  Additionally it is possible to disable the manual control failsafe for this mode, having the vehicle continue on it's set path even if there are no new control inputs.
 
 Manual-Acrobatic
 
-- [Acro](../flight_modes_mc/acro.md) — Manual mode for performing acrobatic maneuvers, such as rolls and loops.
+- [Acro](../flight_modes_mc/acro.md) — Manual mode for performing acrobatic manoeuvrers, such as rolls and loops.
   Releasing the sticks stops the vehicle rotating in the roll, pitch, yaw axes, but does not otherwise stabilise the vehicle.
 
 Autonomous: