docs: add Learning with Squish books landing page

[konjoinfinity]

Summary

• Adds a single-page marketing / email-capture landing page for the "Learning with Squish" children's board book series (Konjo Publishing), at docs/books/index.html
• Deploys via mkdocs' static-file passthrough at squish.run/books (mkdocs copies non-.md files in docs/ verbatim into the built site)
• Hero section with series title/tagline and a placeholder hero illustration box
• 3-book showcase grid (Habits / Counting / Colors) with per-book "Buy on Amazon" buttons wired to a AMAZON_LINKS config object (currently # placeholders)
• Prominent "Get Free Printables!" email capture section (purple gradient card, most visually dominant element on the page) with client-side email validation and a success state
• src query-param attribution tracking: read on load, stashed into a hidden form field, optionally surfaced as a friendly contextual note (e.g. ?src=colors-printables → "Thanks for reading Learning Colors with Squish! 🎨"), and console.log'd as a stand-in for real analytics
• Fully self-contained single HTML file (inline CSS custom properties for the brand palette, inline JS, no build step, no external image dependencies), mobile-first responsive layout

Verified locally with a Playwright smoke test: mobile/desktop screenshots, src param → hidden field + contextual note, invalid-email validation error, and the success state after submit all behave as expected.

Four TODO markers in the file for follow-up work:

1. SWAP-HERO-IMAGE — swap in the real Squish + Lilyan hero illustration
2. SWAP-BOOK-COVER (×3) — swap in real book cover images
3. ESP integration TODO on the /api/subscribe fetch call — wire to a real email service and pass src through as a tagged field
4. AMAZON_LINKS config object at the top of the <script> — drop in real Amazon URLs

Type of change

• Bug fix
• New feature
• Performance improvement
• Documentation update
• Refactor / cleanup

Checklist

• pytest tests/ passes locally (no Python changed)
• ruff check squish/ tests/ reports no errors (no Python changed)
• No hardcoded absolute paths
• No model weights, eval output files, or log files staged for commit
• Changes are scoped to one logical concern (single new static landing page)
• Performance-sensitive changes include a before/after squish bench run (not applicable)

Related issues

N/A

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