Update docs links in ruby files to use .md extension
Issue
Following #419673 (closed) which updated all HAML files to use the .md extension for all links to docs files, we should also do the same for ruby files.
This is the documented standard to follow now: https://docs.gitlab.com/ee/development/documentation/help.html#linking-to-help
This will be enforced in a rubocop rule eventually, as a later iteration of !165516 (merged)