Fix link to Gym compatibility documentation

[LonelyGuy-SE1]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes a broken documentation link that returned a 404 error.
The link has been updated to point to the current Gymnasium compatibility documentation:

https://gymnasium.farama.org/main/content/gym_compatibility/

The second part of the issue seems to be already fixed.

Fixes #1371

Type of change

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• Documentation only change (no code changed)

Screenshots

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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

[Kallinteris-Andreas]

I think it should link to https://gymnasium.farama.org/introduction/migration_guide/