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

introspect/structs.py

@@ -276,6 +276,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='int'),
                  doc='bit flags for enabling optional features',
              ),
+             StructFieldDecl(
+                 name='disableactuator',
+                 type=ValueType(name='int'),
+                 doc='bit flags for disabling actuators by group id',
+             ),
              StructFieldDecl(
                  name='sdf_initpoints',
                  type=ValueType(name='int'),