Document Geo HA setups
GitLab offers both HA and Geo, and the two go together well. Customers will often want a HA setup for their primary and their secondary.
We should include explicit documentation on this, and ensure that any Geo-specific items (such as the tracking database on the secondary) are fully documented / automated.
This will also be useful for the GitLab.com team as they prepare to set up a HA Geo secondary.