chore(v3/docs): fix and improve event system docs

[fbbdev]

Description

This PR fixes and enriches docs for the v3 event system.

Type of change

Please select the option that is 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

Checklist:

• I have updated website/src/pages/changelog.mdx with details of this PR
• My code follows the general coding style of this project
• I have performed a self-review of my own code
• 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

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[leaanthony]

Thanks for working on the event-system documentation. This draft is now out of date and has not had recent activity, so I’m closing it for now to keep the queue current. If the documentation changes are still needed, a refreshed, focused PR with current changelog validation would be welcome.