Documenting the difference in quaternion representation between MuJoCo and Drake

API differences need to be noted when converting rotations between the simulator and the controller. For example,
- MuJoCo API uses an [x, y, z, w] quaternion representation as shown [here](https://mujoco.readthedocs.io/en/stable/modeling.html#frame-orientations)
- Whereas, Drake uses the [w, x, y, z] ordering as described [here](https://drake.mit.edu/pydrake/pydrake.common.eigen_geometry.html?highlight=quaternion#pydrake.common.eigen_geometry.Quaternion)