Rename blueprints: .mdwn → .md
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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.