feat: clean landing page

## Summary

Create a proper Docusaurus landing page that is clean and minimal.

## Inspiration

### [Clojure for the Brave and True](https://www.braveclojure.com/)
- Two-column layout: book cover on one side, call-to-action on the other
- Clear buttons: "Read Free Online" | "Amazon" | "No Starch"
- No sidebar clutter - full-width hero section
- Clean, focused design that emphasizes the free online edition

### [Jest](https://jestjs.io/) (Docusaurus)
- Two-tone dark theme with accent color (green on dark grey)
- 3D animated element spanning the hero section
- "Follow @jestjs_" social button in hero
- Clean navbar: logo, version dropdown, docs links, language, GitHub, dark mode toggle, search
- Feature grid below the hero
- Social proof section ("Who uses Jest?")

## Proposed Design for Effective Shell

### Hero Section
- Two-tone color scheme (works in light/dark mode)
- 3D book cover image on one side
- Tagline + description on the other
- CTA buttons: **Read for Free** | **Amazon** | **No Starch**

### Navbar (top right)
- GitHub stars badge
- Sponsor link
- Dark mode toggle
- Search

### Below the fold
- Brief "What you'll learn" feature cards
- "Who should read this" section

## Current State

The current intro page has sidebar navigation which competes with any hero-style layout. We need a proper Docusaurus landing page component that renders without sidebars.

## Tasks

- [ ] Create `src/pages/index.js` as proper Docusaurus landing page (no sidebars)
- [ ] Move current intro content to `/introduction` or keep in docs
- [ ] Add two-column hero with 3D book cover
- [ ] Style CTA buttons: "Read for Free" (primary), "Amazon", "No Starch" (secondary)
- [ ] Add GitHub stars widget or badge
- [ ] Ensure mobile responsiveness
- [ ] Test in both light and dark modes
- [ ] Consider shell video/gif recording in hero (link to [shellwright](https://github.com/dwmkerr/shellwright))