fix: correct/improve webhook listener event descriptions and examples

[joshtrichards]

☑️ Resolves

• Fix [Bug]: NodeDeletedEvent webhook missing node.id field server#56371
• Fix fix: don't try to get fileid for non exising nodes when serializing events file server#53648 (review)

Summary

• Clarified missing id fields:
  • Added notes on when the id is missing from node event payloads, and recommended using the corresponding Before* event if the id is required.
• Clearer event grouping and applies-to lists:
  • Grouped Node and Calendar Object events by payload format and specified which events use each type.
• Added extensibility notice:
  • Clarified that installed apps can register additional webhook-compatible events, so available events may vary.
• Context for optional app events:
  • Noted that Forms and Tables events apply only if those apps are installed.
• Miscellaneous, Formatting and Sphinx improvements:
  • Made the JSON output prettier
  • Dropped duplicate paragraph
  • Improved blank line usage and literal formatting for better documentation rendering and consistency.

🖼️ Screenshots

[marcelklehr]

Don't these have time and user variables as well?

[marcelklehr]

Looks really good! Thank you @joshtrichards 💙 !