[Reactivity] Live-updating example + docs

[cssbruno]

Add a live-updating example and documentation for realtime reactivity.

Scope:

• Demonstrate a bounded presentation-event driven UI update.
• Document setup, expected behavior, no-JavaScript fallback, and known limits.

Acceptance criteria:

• Example builds and runs with documented commands.
• Docs explain what GOWDK owns versus user Go code.
• Known limitations are listed clearly.