Document implementation of NGINX Ingress Operator

Summary

In https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/issues/179, we decided to deploy the NGINX objects from our GitLab Helm Chart to ensure SSH is supported 'out of the box'. One drawback from that solution is that the 'flavor' of NGINX Ingress Controller used is kubernetes/ingress-nginx, which has no RedHat-certified image for those deploying on OpenShift environments.

We should document the process of using the NGINX Inc. Ingress Operator, which does have a RedHat-certified image.

Relevant notes of the installation process are in this comment.

Acceptance criteria

Documetnation should include:

• how to disable the bundled NGINX objects (via nginx-ingress.enabled=false)
• how to install the NGINX Ingress Operator (perhaps referencing some of the manifests deleted in https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/issues/179)
• how to configure the NGINX Ingress Operator using GlobalConfiguration and TransportServer custom resources to support git over SSH
• A note that these components are a 'feature preview' and are not recommended for production (this is noted in the custom resource links above)