Runtime disabling of actuators according to group.

Added `option-actuatorgroupdisable` attribute and associated `mjOption.disableactuator` integer bitfield, used to disable sets of actuators at runtime according to their group.

- The first 6 actuator groups are toggleable in the `simulate` viewer.
- Minor refactor and cleanup of actuator documentation.

https://youtu.be/H9qG9Zf2W44

Fixes #1092.

PiperOrigin-RevId: 578335600
Change-Id: I4cf663b90ea768e4380acfa2fe3b8c15c7cbb568

src/engine/engine_support.h

@@ -204,6 +204,9 @@ MJAPI void mj_local2Global(mjData* d, mjtNum xpos[3], mjtNum xmat[9],
                            const mjtNum pos[3], const mjtNum quat[4],
                            int body, mjtByte sameframe);
 
+// return 1 if actuator i is disabled, 0 otherwise
+MJAPI int mj_actuatorDisabled(const mjModel* m, int i);
+
 // sum all body masses
 MJAPI mjtNum mj_getTotalmass(const mjModel* m);