Add docs for Runner Operator in disconnected network environments

What does this MR do?

Add documentation that includes the steps required to install the GitLab Runner Operator for Kubernetes in offline environments.

Why was this MR needed?

To support customers using the Runner Operator in offline network environments

What's the best way to test this MR?

What are the relevant issue numbers?

gitlab-org/gl-openshift/gitlab-runner-operator#123

https://gitlab.com/gitlab-com/account-management/western-north-america/google-cloud-gdch/-/issues/29