Docs: refresh the dashboard images for the redesigned widgets

[SandObserver]

Summary

• Replace the Introduction dashboard image with one showing the redesigned System Summary and GitHub widgets. Layout is unchanged.
• Replace the dashboard inside the og:image link preview. Text and frame are unchanged.
• Rename both files to their new content hash and update the references in astro.config.mjs, src/pages/index.astro and docs/index.md.

Testing
npm run build and npm run check pass.

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:	194a519
Status:	✅  Deploy successful!
Preview URL:	https://39bc9279.stackyard-docs.pages.dev
Branch Preview URL:	https://docs-refresh-widget-screensh.stackyard-docs.pages.dev

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