Improvements related to models where joint-actuator relationship is not one-to-one:

- Add `joint-actuatorforcerange` for clamping total actuator force at joints. Add `sensor-jointactuatorfrc` for sensing total actuator forces on a single joint.
See [documentation](https://mujoco.readthedocs.io/en/latest//modeling.html#actuator-force-clamping) for justification and use cases.
- Add simple car model to `model/`.
- Move actuation-related test models into `engine/testdata/actuation/`.

PiperOrigin-RevId: 549355941
Change-Id: I27f6c1f80426d73a2811ef5ae74684a228b2fbd1

src/user/user_objects.h

@@ -296,11 +296,13 @@ class mjCJoint : public mjCBase {
   mjtJoint type;                  // type of Joint
   int group;                      // used for rendering
   int limited;                    // does joint have limits: 0 false, 1 true, 2 auto
+  int actfrclimited;              // are actuator forces on joints limited: 0 false, 1 true, 2 auto
   double pos[3];                  // anchor position
   double axis[3];                 // joint axis
   double stiffness;               // stiffness coefficient
   double springdamper[2];         // timeconst, dampratio
   double range[2];                // joint limits
+  double actfrcrange[2];          // actuator force limits
   mjtNum solref_limit[mjNREF];    // solver reference: joint limits
   mjtNum solimp_limit[mjNIMP];    // solver impedance: joint limits
   mjtNum solref_friction[mjNREF]; // solver reference: dof friction