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/xml/xml_native_writer.cc

@@ -566,6 +566,7 @@ void mjXWriter::OneActuator(XMLElement* elem, mjCActuator* pact, mjCDef* def) {
 
     case mjTRN_SITE:
       WriteAttrTxt(elem, "site", pact->target);
+      WriteAttrTxt(elem, "refsite", pact->refsite);
       break;
 
     case mjTRN_BODY: