Docs: update Consul nodes docs

Context

Follow up from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3645#note_50909359:

I asked:

@dblessing I'm organizing the admin index for https://gitlab.com/gitlab-org/gitlab-ee/issues/4258 and noticed that there's this file: consul.md unlinked from the higher-level doc (https://docs.gitlab.com/ee/administration/high_availability/).

Since you're already working on this, could you please mention it and link it from database.md? Also, the consul nodes would fall into "active/active" or "active/passive" infrastructure? Can you please mention/link the consul.md file from https://docs.gitlab.com/ee/administration/high_availability/README.html?

Drew answered:

@marcia It's interesting that the separate Consul page exists. I'm going to leave that unlinked for now and not make any changes with regard to Consul in this MR. I think we need to decide if we want a dedicated Consul page or if it should be embedded in the database page and make those changes accordingly. At the moment it's a big fragmented if we also link this Consul page you found.

What to do

cc/ @axil @joshlambert

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