docs: modernize README with collapsible sections and tables

- Add modern badges (build, release, stars, .NET version)
- Add Quick Start section at top
- Use collapsible <details> sections for data structures/algorithms
- Use tables for cleaner presentation
- Add complexity info to sorting algorithms
- Add matrix layout for graph variants
- Streamline roadmap to link to TODO.md
- Improve visual hierarchy with section dividers

Author: Ahmad Alhour