Add Categories to External Environments Documentation page

[Kallinteris-Andreas]

Description

• Adds Environments categories for the TOCTree (the thing on the right of the web page)
• fixes a few typos
• moved some envs from gym to gymnasium categories
• Improved Instructions and added a template

Type of change

• 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

Screenshots

TOCtree:

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]

Looks good