Add action masking documentation, update usage scripts

[elliottower]

Description

This PR makes all usage scripts in documentation consistent and uses break (rather than env.step(None), which I personally feel is a bit confusing for new users).

I also added action masking documentation to the AEC API page.

I changed the homepage usage script to use env.action_space(agent).sample() rather than policy(observation, agent) just because it feels like it's cleaner to have code which actually runs, but I can change that back if we want.

Another consistency thing was I added # execute the action in the environment to the usage because that was in Gymnasium and looks nice to just orient users, I can remove that if we don't like it but I think it's a nice touch.

I decided I think this should be removed from both Gymnasium and this (it was added by a new contributor and made the Gym page a bit cluttered, but it's Mark's call, on my other PR in Gymnasium)

Example screenshot of the new homepage:

Action masking documentation:

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.
To upload images to a PR -- simply drag and drop or copy paste.

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

[jjshoots]

LGTM, but since this is documentation, I'd be more confident is someone elsd had a look as well.