Add CleanRL mutli-agent Atari example

[elliottower]

Description

I updated this code in vwxyzjn/cleanrl#408 but I think it'd be good to mirror the code onto here so PettingZoo users see there's a full script with WandB and TensorBoard integration set up already. Changed the number of timesteps to be smaller by default because our CI doesn't take command line args for running tutorials, but other than that it's the same. Looked into embedding an external file using sphinx literalinclude but it seems not to be possible to do it from another separate repo that doesn't use sphinx.

Fixes # (issue), Depends on # (pull request)

Type of change

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• 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

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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 run pytest -v and no errors are present.
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that pytest -v has generated that are related to my code to the best of my knowledge.
• 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