Refactor remove BaseMujocoEnv class

[Kallinteris-Andreas]

Description

Merged MujocoEnv and BaseMujocoEnv into MujocoEnv

also improves some docstrings

BaseMujocoEnv was created to be a super class of MujocoEnv and the old MujocoPyEnv which is no longer developed

validation: for 1mil steps identical behavior was observed https://github.com/Kallinteris-Andreas/gymnasium-changes-validation/tree/main/refactor_mujoco_class

Type of change

• Refactor
• Documentation only change (no code changed)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes