Draft: Update HAML help page links to use .md extension

What does this MR do and why?

Links to help are hard to find and maintain due to the lack of the .md extension in the links from HAML files. Related to the discussions in the related issue, we decided to recommend (and enforce when possible) the use of the file extension. The code is easier to read, and when a TW moves or renames a file, a simple search for the path/name will find it in the code, making it easier to maintain.

This is already the documented recommendation: https://docs.gitlab.com/ee/development/documentation/help.html#linking-to-help

This MR updates the HAML lint that checks help_page_path() and help_page_url() links to enforce .md extensions, and updates all HAML files (with automated find and replace) to use .md in the filename. This is already a huge MR, so addressing other frontend files or ruby files will need to be done in followups.

The documentation link check passes in the pipeline: https://gitlab.com/gitlab-org/gitlab/-/jobs/7672371559

Related to #419673 (closed)

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