docs: overhaul README and add project documentation assets
- Rewrite README.md and README_CN.md with comprehensive project documentation
including core features, architecture overview, benchmarks, tech stack,
quick start guide, and detailed installation instructions
- Add CONTRIBUTING.md with contribution guidelines
- Add architecture.svg and directory-structure.svg diagrams
- Add generated PNG assets for hero banner, core features, pain points,
architecture, and benchmark results
- Add screenshot PNGs for installation guide (PostgreSQL, Neo4j, Alembic,
API docs, frontend UI)
- Replace external GitHub image URLs with local asset references