docs: rewrite README for accessibility and clarity

- Add badges (CI, Python, license, Docker)
- Lead with value proposition and NL example instead of architecture
- Add Key Features section with scannable bullet points
- Move Quick Start before Architecture
- Add 'How It Works' flow diagram
- Consolidate services into a single categorized table
- Collapse project structure into <details>
- Simplify use cases (remove bash noise, focus on pipeline flow)
- Reduce env vars table to essentials (refer to .env.example)
- Use ArrowFlow as project title

Author: VTvito