docs: rewrite README with comprehensive guide and workflow comparison

- Complete rewrite with value proposition, hardware matrix, prerequisites
- Two build paths: primitives vs config-driven pipeline comparison table
- Quick start walkthrough with ConvNeXT (inspect, build, benchmark)
- Built-in model catalog, scope/limitations, roadmap
- Remove SharePoint download link
- Mark cache/doctor/setting as coming soon
- Add BYOM workflow diagram

Author: Zheng Te