feat(site): publish site/ as GitHub Pages start page

[StewAlexander-com]

Summary

Turns site/ into the project's start page and publishes it to GitHub Pages so newcomers land on a polished page that walks them through cloning, installing, and running Python Tutor — without having to read the README first.

Once Pages is enabled on the repo (Settings → Pages → Source: GitHub Actions), the site will be live at:

https://stewalexander-com.github.io/python-tutor/

What changed

• site/index.html — Start section reworked from a single two-command block into three numbered steps (Clone / Install / Run) with their own copy-to-clipboard buttons. Clone uses the HTTPS URL so it works without the GitHub CLI. Added a "common variations" <details> block and a row of prominent links to Repo / README / Issues. <link rel="canonical"> and og:url updated to the Pages URL. Existing dark/amber design and the visual-overflow fix from fix(site): stop Ask tutor popup from obscuring code lab #13 are preserved.
• site/style.css — Adds step-card styling, copy-button styling, the <details> disclosure styling, and a links row. No existing rules were removed.
• .github/workflows/pages.yml — New workflow deploying site/ to GitHub Pages using the official actions/configure-pages@v5, actions/upload-pages-artifact@v3, and actions/deploy-pages@v4 actions. Runs only on pushes to main that touch site/** or the workflow itself, so it doesn't fight the existing CI workflow (ci.yml). Uses contents: read, pages: write, id-token: write permissions and a pages concurrency group with cancel-in-progress: false (the recommended Pages pattern).
• README.md — Adds a top-of-file pointer to the Pages start page and rewrites the "Hero website / start page" section to explain the new URL and deployment.
• site/README.md — Notes the published URL and which workflow deploys it.
• scripts/check_site.sh — Now validates that the clone / install / run commands are present in the HTML, that repo/README/issues links exist, that the copy buttons are wired up, and that .github/workflows/pages.yml is present and references the official Pages actions.

Coexistence with existing CI

The new workflow is in a separate file (pages.yml) with a distinct concurrency group (pages) and path filter (site/**). It does not modify ci.yml. Pushes that don't touch site/ won't trigger a Pages deploy; pushes that don't touch backend/frontend/scripts won't slow CI down.

Testing

• ./scripts/check_site.sh passes locally (all 9 checks ✓)
• python3 -c "import yaml; yaml.safe_load(open('.github/workflows/pages.yml'))" — workflow YAML parses
• Inline copy-button JS passes node --check
• All local asset references in site/index.html resolve on disk
• After merge: enable GitHub Pages → "Source: GitHub Actions" in repo settings, confirm the Pages deploy succeeds and the URL serves the new start page

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