Docs: add the repository social preview card

[SandObserver]

Summary

• Add .github/social-card.png, the 1280x640 repository social preview, showing the built Introduction page inside the landing page's bezel.
• Sits in .github/ rather than public/, so it is versioned without being published on the site, and outside check-dist's scanned roots.

The headline is the repository's own GitHub description. GitHub reads its social preview from repository settings, so the image still has to be uploaded there once.

Testing
All green: npm run build and npm run check both pass. The card was rendered at 400px, the width a phone gives a link preview, to confirm the wordmark, headline and repository path stay legible.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

[cloudflare-workers-and-pages]

Deploying stackyard-docs with    Cloudflare Pages

Latest commit:	01aee49
Status:	✅  Deploy successful!
Preview URL:	https://28462bc0.stackyard-docs.pages.dev
Branch Preview URL:	https://docs-add-social-preview-card.stackyard-docs.pages.dev

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