NGF: document NginxProxy useClusterIP setting

[SidharathBansal]

Proposed changes

Problem: NGINX Gateway Fabric is adding a useClusterIP field to the NginxProxy resource (nginx/nginx-gateway-fabric#5557, resolving nginx/nginx-gateway-fabric#5554). The data plane configuration guide does not yet document it.

Solution: Add a "Route upstream traffic to the Service ClusterIP" section to content/ngf/how-to/data-plane-configuration.md. It explains what the setting does and why (fewer NGINX reloads on backend Pod churn, service-mesh compatibility), gives an NginxProxy YAML example, and documents the fallback cases (headless Services, ExternalName Services, L4/stream upstreams) and the default value.

Testing: Rendered/reviewed the Markdown locally; no functional changes.

Related: nginx/nginx-gateway-fabric#5557, nginx/nginx-gateway-fabric#5554

[salonichf5]

can we target this to release branch for ngf-release-2.7

[SidharathBansal]

Thanks @salonichf5! I've switched the base to ngf-release-2.7. Since the branch was originally cut from main, that introduced conflicts, so I rebased it onto ngf-release-2.7 — the PR now contains only the new "Route upstream traffic to the Service ClusterIP" section and shows as mergeable. PTAL.

[sjberman]

I think we should mention the downside of using this is you lose out on NGINX's load balancing ability, and instead you end up just using k8s default Service load balancing.

[SidharathBansal]

Thanks @sjberman! Added a note calling out the trade-off: enabling useClusterIP makes the upstream a single Service VIP, so you lose NGINX's per-Pod load balancing and instead rely on the Kubernetes Service (kube-proxy) for load balancing. PTAL.

[salonichf5]

overall looks good, just a small comment