Stop exporting MuJoCo-v2 environment bases classes and export MujocoRenderer

[Kallinteris-Andreas]

Description

Fixes #826

as a necessity, it also changes the registration of MuJoCo-v2 environments into

Type of change

• 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

[pseudo-rnd-thoughts]

Do we not export the rendering classes?

[Kallinteris-Andreas]

@pseudo-rnd-thoughts no we do not
metaworld imports and uses MujocoEnv() which uses the render utilities internally.
robotics imports and uses MujocoRenderer() which uses the viewer classes

I have added the exportation of MujocoRenderer() as that may be useful for third_party environments

[pseudo-rnd-thoughts]

Thanks for adding the renderers