Clarify what is Consul and what it can be used for within GitLab

What does this MR do?

This MR clarifies what is Consul (with a short sentence) and what it can be used for within GitLab. Right now, we don't have such thing in the Consul docs and I had a hard time the other day answering this question myself.

There is a thread about this in #support_self-managed.

I'm pinging:

1. @axil - as technical writer (as I think this falls into Distribution group, correct me if I'm wrong).
2. @brodock - as someone who wrote about existing use of Consul at GitLab in past

I would love to get approval/feedback from both of you.

I'm assigning this to @axil for triaging and merging if it's fine. Otherwise, happy to hear what should I do next. 😄