docs: improve README with better structure and quick start guide

- Add Quick Start section at the top for faster onboarding
- Add Prerequisites section (Node.js, pnpm, WordPress requirements)
- Add visual Project Structure tree
- Add Vercel deployment steps (previously only Railway had details)
- Add Troubleshooting/FAQ section for common issues
- Add Contributing guidelines and License section
- Add Customization section for site config and theming
- Remove duplicate Railway button with placeholder
- Consolidate API documentation into concise reference format
- Reduce overall length while improving scanability

Author: claude