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_io.c

@@ -161,6 +161,7 @@ void mj_defaultOption(mjOption* opt) {
   opt->mpr_iterations     = 50;
   opt->disableflags       = 0;
   opt->enableflags        = 0;
+  opt->disableactuator    = 0;
 
   // sdf collisions
   opt->sdf_initpoints     = 40;