Update README with pluggable engine documentation

- Add Quick Start section with installation and basic usage
- Document the pluggable engine architecture
- Add comparison table for available engines
- Explain Docxodus benefits and plan to make it default
- Add API reference section with ComparisonEngine interface
- Update project structure documentation
- Add supported platforms table
- Include development setup and build instructions
- Add roadmap section
- Add CI badge and other status badges
- Link to Docxodus, Open-Xml-PowerTools, and other resources

Author: claude