Document installation in Docker swarm mode

See the general Documentation guidelines https://docs.gitlab.com/ee/development/doc_styleguide.html

What does this MR do?

Documents installation in Docker swarm mode as a stack, including Docker secrets and configs.

Related issues

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20625
• !2647 (closed)

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page
• Change the header tag to h5, #####

Moving docs to a new location?

See the guidelines: https://docs.gitlab.com/ee/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.