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

test/header_test.cc

@@ -31,6 +31,15 @@ namespace {
 
 using HeaderTest = MujocoTest;
 
+TEST_F(HeaderTest, IntsHave4Bytes) {
+  EXPECT_EQ(4, sizeof(int));
+}
+
+TEST_F(HeaderTest, IntsHaveAtLeast31Bits) {
+  int shift_left_30 = 1 << 30;
+  EXPECT_GT(shift_left_30, 0);
+}
+
 TEST_F(HeaderTest, EnumsAreInts) {
   EXPECT_EQ(sizeof(mjtDisableBit),      sizeof(int));
   EXPECT_EQ(sizeof(mjtEnableBit),       sizeof(int));