Update documentation links linter

What does this MR do and why?

Contributes to #345474 (comment 1447561145)

• Disallow .md extension usage (as recommended by https://docs.gitlab.com/ee/development/documentation/help#linking-to-help)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.