Update environment Wiki with Mujoco environments

[zuoxingdong]

It could be better to update the Wiki for environment with Mujoco ones, e.g. Swimmer etc.

In addition, maybe better to have explanation for each dimension of action space, e.g. first dimension is position, second dimension is velocity.

[madvn]

I second this request for MuJoCo envs - see #106

[Amritpal-001]

@zuoxingdong I compiled documentation on Mujuco's Fetch environments It doesn't cover Swimmer, but Most of the principles are common amongst mujoco environment, so this might be helpful. I have tried to cover the observation and action variables, reward function, and comparison among all 4 fetch environments. https://link.medium.com/CV6la7YfV7

I hope it's helpful.

[jkterry1]

PR #2762 is about to be merged, introducing V4 MuJoCo environments using new bindings and a dramatically newer version of the engine. If this issue still persists with the V4 ones, please create a new issue for it.