Improvements to `simulate`:
Improvements to `simulate`:

- Add '[' and ']' to cycle through fixed cameras.
- Remove options to step backwards or step 100 steps forwards or back.
- Add missing elements from built-in help.
- Reorder help strings.

PiperOrigin-RevId: 436497103
Change-Id: I9f910ea328c859239b46c9d7a47e4ee16a43f1ad

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco