Clarify how to link your custom domain with Pages in our guides
In the past two weeks there've been two reports on this issue.
- https://gitlab.com/gitlab-com/support-forum/issues/569#note_39961589
- https://gitlab.com/gitlab-com/support-forum/issues/2498#note_41500106
We need to be more clear about the requirement to associate your custom domain in Pages settings in our guides. This is covered by https://docs.gitlab.com/ee/user/project/pages/introduction.html#add-a-custom-domain-to-your-pages-website, so we should either make that doc more apparent in the guides or link to that section when it comes to associate the custom domain in Pages settings.