Mujoco sim upgrades

[Zolkin1]

Updated some features in the Mujoco simulation:

• Removed the need to specify nu in the yaml, instead we now read that from the Mujoco model. This was annoying because if you forgot to update the yaml it was hard to figure out what the issue was.
• Removed the need to specify the sim timestep in the yaml, instead we now read that from the Mujoco model. In the future we can add back the option to overwrite the Mujoco time step with what the user puts in the yaml, but for now that seems unnecessary.

Updated the docs to be more accurate.

[alberthli]

lgtm