Update more docs for new architecture

[sjberman]

There are a couple of docs that need to be updated to reflect the new architecture.

Specifically:

• https://docs.nginx.com/nginx-gateway-fabric/traffic-management/basic-routing/: an image references traffic flowing through the nginx-gateway Pod in the nginx-gateway namespace, which is no longer the case. It's not the nginx Pod in the Gateway's namespace
• https://docs.nginx.com/nginx-gateway-fabric/traffic-management/advanced-routing/: Same issue

[github-actions]

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