Create crosslink between email pages
This small merge request came about because of gitlab-org/growth/team-tasks#364 (closed) - I was floundering in a docset not my own, and I kept finding the wrong page for information about emails and GitLab. This MR crosslinks the two pages. We want to make it as easy as possible for users who want to opt out, to do so. It's good manners.
Related to gitlab-org/growth/team-tasks#364 (closed)