Explain how to import project template
What does this MR do and why?
Self-managed instances can contain outdated version of the cluster management template. We have a note about importing it via the URL in https://gitlab.com/gitlab-org/gitlab/blob/master/doc/user/clusters/migrating_from_gma_to_project_template.md#L15. This MR also adds the same information to How to use this template