Add readonly connections documentation for Agents SDK

Adds comprehensive documentation for the new readonly connections feature introduced in PR #610.

Key additions:
- Server-side methods: shouldConnectionBeReadonly(), setConnectionReadonly(), isConnectionReadonly()
- Client-side API: onStateUpdateError callback for error handling
- Multiple usage examples covering common scenarios (query params, RBAC, admin dashboards, dynamic permissions)
- Implementation details including SQL persistence and hibernation support
- Best practices for authentication, user feedback, and access control

This feature allows restricting certain WebSocket connections from modifying agent state while still allowing them to receive state updates and call RPC methods.

Related PR: cloudflare/agents#610

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: github-actions[bot]