Cartesian end-effector control using site transmission.

- Added `refsite` attribute to actuators with `site` transmission.
- Such actuators now have a well defined length and can be used for e.g., Cartesian end-effector control.
- Added example models and documentation describing the new feature.

PiperOrigin-RevId: 471561218
Change-Id: I538f09af9600d5c53992c4a9fed325648aab4fa3

src/user/user_objects.h

@@ -863,6 +863,7 @@ class mjCActuator : public mjCBase {
   std::vector<double> userdata;   // user data
   std::string target;             // transmission target name
   std::string slidersite;         // site defining cylinder, for slider-crank only
+  std::string refsite;            // reference site, for site transmission only
 
  private:
   mjCActuator(mjCModel* = 0, mjCDef* = 0);// constructor