docs: add naxytra discovery note

[naxytra]

Summary

• Adds a public MCP discovery note to the AutoGen community projects page.
• Points readers to the canonical naxytra discovery index, A2A card, and live MCP endpoint.

Why

• AutoGen users looking for community MCP integrations should have a concrete public example to reference.
• This keeps the community discovery page aligned with the live public entry surfaces without exposing private code.

Links

• https://www.naxytra.com/v1/agent-discovery-index
• https://www.naxytra.com/.well-known/agent.json
• https://xytara-mcp-executor.onrender.com/mcp