Add landing page for devup-api

Create a comprehensive landing page inspired by devup-ui, featuring:
- Hero section with key value propositions (OpenAPI-driven, Fetch-compatible, Zero Runtime)
- Features section highlighting 6 core capabilities
- Code examples demonstrating installation, basic usage, React Query integration, and type references
- Packages overview showcasing all 9 devup-api packages
- Footer with company information and resource links
- Fully responsive design with modern styling

Author: claude