Update extension for docs links in ci ruby files

Marcel Amiraultrequested to merge
ui-doc-link-md-extension-ruby-2 into master

What does this MR do and why?

We should use .md in all links to markdown documentation files for better maintainability, which is the documented standard to follow now: https://docs.gitlab.com/ee/development/documentation/help.html#linking-to-help

This MR updates ruby files in **/ci/

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Edited by Marcel Amirault

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