Publish the site as the Lightcone Research Stack docs

[EiffL]

Summary

Publishing

• .github/workflows/docs.yml replaces build.yml. Every PR still gets a strict zensical build. On main (push or manual run), the same build is uploaded and deployed to GitHub Pages.
• site_url = "https://docs.lightconeresearch.org/", so canonical links and the sitemap use the real domain.

Lightcone Research Stack branding

• The site name and description are now "Lightcone Research Stack".
• On the home page:
  • The intro and alpha warning now describe the whole stack, and the warning points to the LightconeResearch org for issues.
  • "Two libraries, one toolchain" is now "Components of the stack".
  • The contributor card says it covers lightcone-cli's internals.
• Pages about the CLI itself are unchanged.

README and license

• README.md is rewritten as a landing page for readers: what the stack is, a link to the site, where to start, the components, and where to report problems.
• LICENSE is lightcone-cli's BSD 3-Clause (CNRS and The Regents of the University of California). The imported pages were already published under it.

Repo settings (already applied)

• Pages is enabled with Source: GitHub Actions.
• Custom domain is docs.lightconeresearch.org, with the HTTPS certificate approved and HTTPS enforced. The DNS record already pointed at GitHub Pages, but no repo claimed the domain, so it was returning a 404.

The first deploy runs when this PR is merged.

Test plan

• uv run zensical build --clean --strict: no issues; canonical and sitemap URLs use https://docs.lightconeresearch.org/
• actionlint passes on the workflow
• Every docs link in the README resolves to a built page
• After merge: the deploy job succeeds and https://docs.lightconeresearch.org serves the site

🤖 Generated with Claude Code