Update documentation for GitLab UI integration testing with the core product

This issue is being opened to update documentation how we use the forking workflow to test GitLab UI changes in the core product without including other commits that may exist between the published and current version of GitLab UI.

Having recently worked on my first GitLab UI integration with a decently sized changeset, I'd like to capture my insights and terminal commands to help other GitLabbers make updates more smoothly.