ControlAllocator: Periodically spin MC motors to shed ice (#26322)

This feature periodically spins the unused motors on VTOL airframes, to
break off ice that has built up in the motor while it is still feasible
to do so. Each ice shedding event sends a motor ouput of 0.01 to the
unused motors for 2 seconds.

Configured by `CA_ICE_PERIOD`, the duration of the entire cycle. Set to
0 to disable.

docs/en/config_vtol/index.md

@@ -9,3 +9,4 @@ Then perform VTOL-specific configuration and tuning:
 - [Back-transition Tuning](../config_vtol/vtol_back_transition_tuning.md)
 - [VTOL w/o Airspeed Sensor](../config_vtol/vtol_without_airspeed_sensor.md)
 - [VTOL Weather Vane](../config_vtol/vtol_weathervane.md)
+- [Ice Shedding](../config_vtol/vtol_ice_shedding.md)

docs/en/config_vtol/vtol_ice_shedding.md

@@ -0,0 +1,20 @@
+# VTOL Ice Shedding feature
+
+## Overview
+
+Ice shedding is a feature that periodically spins unused motors in fixed-wing
+flight, to break off any ice that is starting to build up in the motors while it
+is still feasible to do so.
+
+It is configured by the paramter `CA_ICE_PERIOD`. When it is 0, the feature is
+disabled, when it is above 0, it sets the duration of the ice shedding cycle in
+seconds. In each cycle, the rotors are spun for two seconds at a motor output of
+0.01.
+
+:::warning
+When enabling the feature on a new airframe, there is the risk of producing
+torques that disturb the fixed-wing rate controller. To mitigate this risk:
+ - Set your `PWM_MIN` values correctly, so that the motor output 0.01 actually
+   produces 1% thrust
+ - Be prepared to take control and switch back to multicopter
+:::