Rename blueprints: .mdwn → .md
Rationale:
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Right now:
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These pages' Markdown will be rendered when browsing our repository in the GitLab web interface
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The .md extension is more common nowadays: GitHub, GitLab, and various other major players have settled on it. Let's get used to it.
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If we decide to migrate our blueprints to GitLab (#18079 (closed)), this will be a necessary first step.