Rewrite TLS for Functions guide to use Gateway API and HTTPRoutes

Replace the FunctionIngress/ingress-operator approach with
Kubernetes Gateway API. The updated guide covers adding Gateway
listeners per function domain, creating HTTPRoutes that rewrite
paths to /function/NAME/, and cert-manager integration for TLS.

Includes implementation-specific tabs for Envoy Gateway (regex
rewrite workaround) and standard Gateway API (ReplacePrefixMatch).

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

Author: welteki