Add link from docs front page to release notes

It's difficult to identify recent changes to our documentation. On its own, our users who have to scan the equivalent of 2 copies or "War and Peace" to find what's changed, from release to release.

Fortunately, we have https://gitlab.com/gitlab-org/gitlab/-/releases, which identifies those changes. But we don't currently include that in our documentation. I think we should, on our "front page", at https://docs.gitlab.com/, something like (from the build of this MR):

Thanks @sselhorn for noting that the Activate GitLab EE entry is the least popular of the "Popular topics". With that in mind, this MR replaces that topic with a link to our Release Notes, as shown in the screenshot.

@axil @eread : I'd appreciate your thoughts. For previous discussion, see this part of the relevant Slack thread: https://gitlab.slack.com/archives/CDWGE0DDY/p1605887709183400?thread_ts=1605742591.175100&cid=CDWGE0DDY

closes technical-writing#303