style(docs): improve visual design — hero, cards, buttons, typography, spacing
1. Hero section: larger (max-width 800px), bigger heading (3rem), subtle
background gradient, increased padding, social proof slot
2. Section spacing: h2 bottom borders, 2.5rem top margin, hr 3rem margin,
alternating section backgrounds on homepage
3. Cards: background color, subtle default shadow, lift on hover (translateY),
border highlights primary color on hover
4. Typography: base 0.78→0.82rem, h2 larger (1.4em) with 700 weight
5. Buttons: larger (0.9rem, more padding), 8px radius, primary shadow,
lift on hover for both primary and outline
6. Mobile: responsive hero adjustments for smaller screens
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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