Modernize landing page with shared 21no.de brand

Refactor docs/index.html to adopt the shared-web-assets design system
served from assets.21no.de instead of a bespoke inline stylesheet:

- Link shared fonts + design tokens, base, nav, buttons, hero, cards,
  and footer CSS (single source of truth across 21no.de properties)
- Rebuild nav, hero (centered with badge, stats, and a terminal demo
  in the brand code-block), feature/architecture/ecosystem cards,
  CTA, and footer using brand component classes
- Replace Google Fonts with the brand's self-hosted Outfit + JetBrains
  Mono and align theme-color with the brand background
- Keep page-specific styles (install steps, terminal, CTA) inline per
  the shared-web-assets convention
- Preserve all SEO metadata, Open Graph/Twitter tags, and structured data

Net: -1136/+302 lines

Author: claude