Add environments demo video to homepage

[elliottower]

Description

This PR adds a video I made demonstrating PettingZoo environments, to match the similar video for Gymnasium: https://gymnasium.farama.org/main/

I took a few of them from https://umshini.ai/, some from the default image previews (pistonball), and recorded some locally (knights archers zombies, chess)

Screenshot of video on page:

Full video:
https://user-images.githubusercontent.com/32176771/233870105-b7fa0fed-759b-4215-ac1a-b38b22546893.mp4

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

Type of change

Please delete options that are not relevant.

• 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

Please attach before and after screenshots of the change if applicable.
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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