Horizontal HA doc installation steps

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Follow-on to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9174

Add installation instructions for Horizontal in the same fashion as we have now for Basic Scaling

• Link to and update PostgreSQL and Redis HA sections
• Move Consul to its own page. This will help reduce the size and scope of the PG page. We may also use Consul for other things in the future so this will set us up for that.
• In conjunction with above, figure out how to represent the combined Consul/Sentinel nodes that we recommend for horizontal. Would it make sense to have a page that contains both Sentinel and Consul instructions to keep things 'DRY'?