docs(dev/events): modernize / expand event system docs

[joshtrichards]

☑️ Resolves

The existing Events chapter was showing its age and in need of a refactor.

It was a bit confusing in some spots, wasn't up-to-date with current best practices, and lacked coverage of the dispatcher.

This PR modernizes and expands the documentation for Nextcloud Events:

• adds emission/dispatch coverage
• clarifies listener registration / instantiation / handler execution lifecycle
• aligns docs/examples with current best practices - e.g.,
  • use the OCP Event Dispatcher and IBootstrap patterns
  • current PHP best practices
• adds additional code examples:
  • e.g., defining events, registering listeners, and emitting/dispatching events, usage of event payloads, and DI
• improves coverage of event payloads
• de-emphasize legacy approaches, referencing them only for migration purposes
  • e.g., hooks, public emitters, pre-IBootstrap listener patterns
• improves clarity throughout the documentation
  • e.g., correcting typos, clarifying ambiguous statements, and removing (most - ahem) informal phrasing
• new table of contents

These changes ensure app authors have clear, up-to-date, and actionable guidance for both consuming and producing events in Nextcloud apps.

🖼️ Screenshots

[joshtrichards]

Revised further:

• clarified various confusing bits
• fixed some inaccurate spots
• fixed some broken examples
• integrated earlier review feedback
• added some additional events and scenarios