Add comprehensive TLS guide for AWS EKS with Gateway API

- Document complete setup for NLB with Envoy Gateway using Gateway API
- Cover both ALB (L7) and NLB (L4) approaches for different use cases
- Include cert-manager configuration for Gateway API resources

Signed-off-by: Han Verstraete (OpenFaaS Ltd) <han@openfaas.com>

Author: welteki