Attach gemstack.land custom domain to the docs site

[suleimansh]

The docs site is live on GitHub Pages at https://gemstack-land.github.io/gemstack/ (deployed via deploy-docs.yml). Once the gemstack.land domain is in place (transfer Hover -> Cloudflare in progress), cut the site over to it.

Cutover checklist

• DNS: point gemstack.land at GitHub Pages (CNAME/ALIAS to gemstack-land.github.io, or per the hosting decision below)
• Set the Pages custom domain (gh api -X PUT repos/gemstack-land/gemstack/pages -f cname=gemstack.land) and wait for GitHub to provision TLS
• Drop DOCS_BASE=/gemstack/ from .github/workflows/deploy-docs.yml so the VitePress base returns to /
• Re-swap the interim github.io links to gemstack.land:
  • rudder docs/guide/ai.md + docs/guide/mcp.md (currently github.io, rudder PR #1465)
  • the package README "Docs" column links
• Verify the deep pages resolve at the new root

Out of scope (decided in Discord)

The governance questions (whose Cloudflare account owns the domain, GitHub Pages vs Cloudflare Pages, who controls deploys) are being settled in chat. This issue only tracks the mechanical cutover once that's resolved.

Non-urgent: the github.io URL works today and will redirect once the custom domain is attached.

[suleimansh]

Decisions settled with the Vike team:

• Registrar: transfer gemstack.land to Cloudflare (more trustworthy than small registrars). The domain-owning Cloudflare account invites the other to manage, no password sharing.
• Hosting: keep the docs on GitHub Pages (current setup), do NOT move to Cloudflare Pages for now. Simple and already working (same approach as vike.dev).
• Deploys: keep the existing org-repo workflow that ships docs on merge to main (anyone with merge rights ships).
• No rush on the transfer, site is already up.

Remaining work here: transfer the domain to Cloudflare, then attach gemstack.land as the custom domain on the existing Pages deploy.