Docs: Update New Events Emitter

[eonarheim]

Context

Currently the page is empty! 😱

Proposal

• once, on, off examples
• Talk about strongly typed emitters with examples
• Show how to manage subscriptions
• Piping events from one to another
• Talk about how events work in excalibur

[github-actions[bot]]

This issue hasn't had any recent activity lately and is being marked as stale automatically.