Added documentation for permalinks to most recent build artifacts

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Add missing documentation identified in #27361 (closed).

• Documented how to download single files from most recent artifacts
• Documented how to browse most recent artifacts

Moving docs to a new location?

No.

Closes #27361 (closed)