Add GCP + cert-manager TLS setup and production hardening tips to Secure Gateways guide

I’m a network security architect currently learning and exploring Envoy Gateway. I noticed the [Secure Gateways]( https://gateway.envoyproxy.io/latest/tasks/security/secure-gateways/ ) page is a great starting point but could be enhanced for broader cloud users, especially GCP. Here are some suggestions:
1. Add GCP-specific notes on:
• Using cert-manager with Let’s Encrypt or Google-managed certs
• Ingress Gateway DNS setup with Cloud DNS
2. Production readiness tips:
• Avoid using self-signed certs (ok for demo, but not for production)
• Mention use of mTLS between gateway and backend (optional)
3. Architecture diagram suggestion:
• A diagram showing client → TLS termination at Envoy Gateway → Kubernetes service/backend

I’d be happy to collaborate on improving this section. Thanks for your awesome work!