Add documentation for readonly connections

Synced from cloudflare/agents PR #610

Introduces comprehensive documentation for the readonly connections feature,
which allows restricting certain WebSocket connections from modifying agent
state while still allowing them to receive state updates and call RPC methods.

Includes:
- Server-side API methods (shouldConnectionBeReadonly, setConnectionReadonly, isConnectionReadonly)
- Client-side error handling (onStateUpdateError callback)
- Usage examples for common scenarios (view-only modes, role-based access, dynamic permissions)
- Best practices for authentication, user feedback, and logging
- Details on persistence across hibernation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: github-actions[bot]