Remove auto-correctable links from rubocop todo

What does this MR do and why?

Fix links identified by new cop added in !165516 (merged)

As a first step, this only addresses auto-correctable rubocop violations (missing .md extension). I auto-corrected them, and then updated code/specs to match, when needed.

In some cases, a ruby spec was related to frontend .vue or .js files, so I updated those as well so that all the specs would pass.

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