Updating MuJoCo docstrings

[Markus28]

This is a WIP

Imo a big issue is that it's not clear how v2 and v3 relate and which version is actually being documented. Am I right in assuming that

1. We concentrate on v3?
2. The only difference between v3 and v2 is configurability? I.e. the default configuration of v3 is equivalent to v2? (That's what the current version info seems to imply)

Changes so far:

General Changes

• Rephrase action space
• In "Observation Space": "state space" -> "observations": This seems to be a recurring phrasing in the MuJoCo docs. My reason for changing this is two-fold: If we are being pedantic, the state space does not consist of positional values of different body parts, but of states. Secondly, there is a slight distinction between states and observations in the setting of POMDPs and we are mostly concerned about observations.
• Change the tables for observation space to illustrate exclude_current_positions_from_observation=True and discuss exclude_current_positions_from_observation=False as special case
• Talk about weights for reward terms
• Update names of reward terms to match code
• Note that individual reward terms are returned in info
• Talk about terminate_when_unhealthy
• Currently, the documentation is encouraging users to modify the XML assets that come with Gym. I don't think that's a good idea, especially since v3 allows to specify the path to a custom MuJoCo model.
• Give a list of kwargs for make

Ant

• Change y-coordinate to z-coordinate in termination criterion

Half Cheetah

• Changed y-coordinate to z-coordinate. TODO: Is this correct?

Humanoid

• Question: Is there any difference between "control" and "action". Why use the term "control"?
• I cannot claim to understand cinert, cvel etc.

HumanoidStandup, InvertedDoublePendulum, InvertedPendulum

• Remove v3 from version info

There are probably a bunch of changes that I forgot to mention in the list above

[jkterry1]

I'm merging this so it's in the docs string for the release, please create another PR for any addition changes agreed upon