docs: Add comprehensive Astro.js implementation guide

- Complete technical guide for building PWB frontend with Astro.js
- Detailed comparison with Next.js (8.5/10 vs 9.25/10)
- Why Astro is ideal for property listing sites (content-heavy)
- Complete architecture diagrams and code examples
- Performance targets: 100/100/100/100 Lighthouse scores
- Project structure, setup, and deployment instructions
- API integration examples with PWB backend
- SEO implementation (sitemap, robots.txt, structured data)
- Dynamic theming system from backend API
- React islands architecture for selective interactivity
- 8-week implementation timeline
- Dokku deployment guide with multi-tenant support

Key advantages:
- Zero JavaScript by default (fastest performance)
- 30-50% faster than Next.js
- Perfect for 80% content / 20% interactive ratio
- Simpler mental model (like ERB/Liquid templates)
- Framework flexible (use React only where needed)

Author: etewiah