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

test/engine/testdata/actuation/damper.xml

@@ -0,0 +1,19 @@
+<mujoco>
+  <compiler autolimits="true"/>
+
+  <option integrator="implicitfast"/>
+
+  <worldbody>
+    <geom type="plane" size="1 1 .01"/>
+    <light pos="0 0 2"/>
+    <body pos="0 0 .3">
+      <joint name="hinge" axis="0 1 0"/>
+      <geom type="capsule" size=".01" fromto="0 0 0 .2 0 0"/>
+      <geom size=".03" pos=".2 0 0"/>
+    </body>
+  </worldbody>
+
+  <actuator>
+    <damper name="damper" joint="hinge" kv="10" ctrlrange="0 1"/>
+  </actuator>
+</mujoco>