fix(markdown): point "Edit this page" at the upstream repo

[ryzrr]

Summary
It now carries a source frontmatter , so that the edit the page is pointing to the correct path

What kind of change does this PR introduce?
fix

Did you add tests for your changes?
no

Does this PR introduce a breaking change?
no

If relevant, what needs to be documented once your changes are merged or what have you already documented?
no

Use of AI
slightly

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[vercel]

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Project	Deployment	Actions	Updated (UTC)
webpack-doc-kit	Ready	Preview, Comment	Jul 25, 2026 6:08pm

[github-actions]

⚡ Lighthouse report

Audited the Vercel preview deployment: https://webpack-doc-a1vixhq5u-openjs.vercel.app

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🟢 92	🟢 96	🟢 100	🟢 100	🔗
/about	🟢 99	🟢 97	🟢 100	🟢 100	🔗
/about/sponsors	🟢 98	🟢 95	🟢 96	🟢 100	🔗
/blog	🟢 99	🟢 96	🟢 100	🟢 100	🔗
/blog/posts/2026-02-04-roadmap-2026	🟢 99	🟢 93	🟢 100	🟢 100	🔗
/guides/getting-started	🟢 95	🟢 97	🟢 100	🟢 100	🔗
/docs/api/v5.x/options	🔴 68	🟢 96	🟢 100	🟢 100	🔗
/docs/loaders/css-loader	🟢 99	🟢 97	🟢 100	🟠 83	🔗

Scores are advisory (warn at 90); they do not fail the build. See .lighthouserc.json.