Docs: point the demo links at demo.sandobserver.com

[SandObserver]

Summary

• Replace the three stackyard-demo.onrender.com links with
  demo.sandobserver.com: the landing page hero button, the landing page
  footer, and the introduction page.

The cold-start note is unchanged. It is the same free tier and still sleeps
when idle, so the "first visit can take up to a minute" wording and the matching
troubleshooting entry both still hold.

Testing
npm run build and npm run check pass. Verified the new host returns 200 with
a valid certificate, and that no reference to the old host remains in any
tracked file.

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:	76491a6
Status:	✅  Deploy successful!
Preview URL:	https://2e301b46.stackyard-docs.pages.dev
Branch Preview URL:	https://docs-demo-custom-domain.stackyard-docs.pages.dev

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