Document MLX environment variables

[XXXXRT666]

Proposed changes

• Add a centralized reference for MLX runtime environment variables.
• Link existing usage documentation to the corresponding entries.

Checklist

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[zcbenz]

Thanks for putting these together!