feat: comprehensive SEO optimization (#66)

- Add title and description frontmatter to index.md (en + zh-cn) — fixes broken meta descriptions
- Add hero section with heading, description, CTA button, and alt text on homepage
- Add proper description frontmatter to all 40 English + 40 Chinese doc pages
- Fix tagline from keyword-stuffed list to natural sentence
- Add og:image, twitter:card, twitter:image meta tags
- Add robots/index and googlebot/index meta tags
- Add JSON-LD structured data (Book schema) via custom plugin
- Add robots.txt in static/ with AI crawler allowances
- Enable sitemap lastmod dates for better crawl scheduling
- Add structured data plugin with bilingual Book schema support

Build verified: npm run build succeeds for both en and zh-cn locales.

Author: luckrnx09