GitLab

Rationale:

 - Right now:

   - These pages' Markdown will be rendered when browsing our
     repository in the GitLab web interface

   - The .md extension is more common nowadays: GitHub, GitLab, and various
     other major players have settled on it. Let's get used to it.

 - If we decide to migrate our blueprints to GitLab (#18079),
   this will be a necessary first step.

refs #17811

From her message to tails-project@, it seems that ignifugo did not
understand that she could copy this information herself.
A clarification about where this info lives might help.