GitLab HA docs index page

There's some confusion about GitLab HA. First off what it is, and whether it's available outside EE. We're resolving that partially with gitlab-com/www-gitlab-com!6605 (diffs), but we still have CE docs about HA (starting at https://docs.gitlab.com/ce/administration/high_availability/README.html). Presumably these should be removed or somehow clarified that they're not the officially supported way.

Also, we have some likely stale content on GitLab University (https://docs.gitlab.com/ce/university/high-availability/aws/) and YouTube (https://www.youtube.com/watch?v=36KS808u6bE). If it's still valid, then perhaps we keep it, at least for now. It's often not worth re-recording everything constantly. But if it is valid, we should consider linking to the content from a single master HA index. Recently, some of the other topic pages in docs have been rewritten in a new format that provides room to link to videos and third-party content. See https://docs.gitlab.com/ee/ci/ for an example. Maybe GitLab HA deserves similar treatment.

/cc @joshlambert @sytses