Fix GitHub Pages deployment for Hugo

[besendorf]

What changed

• Build the Hugo site in GitHub Actions and upload the generated public/ directory as a Pages artifact.
• Deploy with GitHub's actions/deploy-pages workflow and the required pages: write and id-token: write permissions.
• Keep pull requests build-only; deploy only from main, with a manual workflow_dispatch option.

Why

The existing workflow pushed output to gh-pages with GITHUB_TOKEN. GitHub documents that workflow commits made with GITHUB_TOKEN do not trigger a GitHub Pages build, which left the published site stale. This workflow uses the supported Pages artifact deployment path.

Required setting

Before deployment, set Settings → Pages → Build and deployment → Source to GitHub Actions.