Add control values to keyframes.

- Add basic test for keyframes.
- Add missing documentation for keyframe mocap positions and quaternions.

PiperOrigin-RevId: 459021649
Change-Id: I91cf7ecbddc6262e8c72a868eeb82d627f389fb3

src/user/user_objects.h

@@ -951,6 +951,7 @@ class mjCKey : public mjCBase {
   std::vector<double> act;        // act
   std::vector<double> mpos;       // mocap pos
   std::vector<double> mquat;      // mocap quat
+  std::vector<double> ctrl;       // ctrl
 
  private:
   mjCKey(mjCModel*);              // constructor