WIP: Move unicorn settings doc from Omnibus to CE

Open Achilleas Pipinellis requested to merge unicorn-settings-doc into master

What does this MR do?

Move Unicorn settings from Omnibus to CE.

Why was this MR needed?

Because this is also relevant to source installations and we need to have one document for both source and Omnibus to avoid duplication.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/18617