Document operator properties, custom installation and permissions

What does this MR do?

Documents all the properties of the Runner Operator for OpenShift. Also adds some useful examples.

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

Closes gitlab-org/gl-openshift/gitlab-runner-operator#29 (closed), gitlab-org/gl-openshift/gitlab-runner-operator#6 (closed), gitlab-org/gl-openshift/gitlab-runner-operator#20 (closed), gitlab-org/gl-openshift/gitlab-runner-operator#16 (closed)