Migrate GitHub Pages publishing to GitHub Actions workflow

[mmcky]

Background

The site is currently published via the legacy GitHub Pages build (Pages → Source: main branch, / root). This means production builds use the GitHub Pages gem sandbox, which is pinned to Jekyll 3.10 and libsass (via jekyll-sass-converter 1.x).

In #194 the local Gemfile was bumped to jekyll ~> 4.4, but that version is only used locally and on the Netlify deploy preview — production still runs on the older sandboxed Jekyll. This was discovered when the new SCSS used @use directives (Dart Sass), which work locally but would fail on legacy GH Pages. As a workaround, 65163f9 reverted those back to @import.

Goal

Switch to a GitHub Actions deploy workflow so production uses the same Jekyll version as the Gemfile, and so we can use modern features (Dart Sass @use/@forward, newer Liquid filters, etc.) without surprises.

Proposed approach

1. Pages settings → change Source from "Deploy from a branch" to "GitHub Actions" (https://github.com/QuantEcon/website/settings/pages)
2. Add .github/workflows/deploy.yml using the standard pattern:
   • actions/checkout
   • ruby/setup-ruby with bundler-cache: true
   • actions/configure-pages
   • bundle exec jekyll build (with JEKYLL_ENV=production)
   • actions/upload-pages-artifact
   • actions/deploy-pages
3. Trigger on push to main (and workflow_dispatch)
4. Keep the existing PR build.yml for preview validation, or merge them

After migration we can

• Restore @use in assets/main.scss and remove the Dart Sass @import deprecation warnings
• Use any Jekyll 4.x-only filters (group_by_exp, etc.) safely
• Drop reliance on the github-pages gem set entirely

Verification

• Confirm CNAME (quantecon.org) is preserved
• Confirm redirects in pages/ still resolve
• Verify all collections (_posts, _lectures, _team-members, _projects) build identically
• Check sitemap.xml and feed.xml output

References

• https://docs.github.com/en/pages/getting-started-with-github-pages/configuring-a-publishing-source-for-your-github-pages-site#publishing-with-a-custom-github-actions-workflow
• Discovered in Redesign and modernise quantecon.org #194 review