Create a High Availability and Disaster Recovery Topic

There are various locations where HA documentation exist. We need to consolidate this, and also remove some old pages.

• University does not link to the HA AWS guide at https://docs.gitlab.com/ce/university/high-availability/aws/
• University does link to https://about.gitlab.com/high-availability, which is extremely outdated. This page should be removed from https://about.gitlab.com and relevant pieces should be consolidated into https://docs.gitlab.com/ce/high-availability.
• AWS and other HA guides should be linked from University, and probably cross-linked from the HA docs page.

cc/ @axil @seanpackham What do you think?

I need to make some updates to these docs so I will try to tackle some of these things.