pWIP: Add configuration section to docs for runner Operator on OpenShift

What does this MR do?

Adds a configuration section to the runner docs that guides how to configure the runner on OpenShift.

Why was this MR needed?

This MR is required to provide additional guidance to the customers adopting the GitLab Runner Operator on OpenShift.

What's the best way to test this MR?

What are the relevant issue numbers?

gitlab-org/gl-openshift/gitlab-runner-operator#29 (closed) gitlab-org/gl-openshift/gitlab-runner-operator#21 (closed)