[Realtime] Realtime addon docs

[cssbruno]

Document the realtime addon surface.

Scope:

• Explain dependency boundaries, configuration, SSE default behavior, WebSocket opt-in behavior, and deployment caveats.
• Keep realtime transport separate from M14 live DOM reactivity.

Acceptance criteria:

• Docs describe setup and supported transport behavior.
• Docs state what remains planned for live UI reactivity.
• Example commands are runnable or explicitly marked future work.