docs: update READMEs to reflect Hub-based interactive sample browser (#106)

- Replace old multi-project architecture with Hub-based interactive sample browser
- Update .NET version badge from 8.0 to 10.0
- Add interactive menu screenshot and sample catalog table
- Document ISample interface and auto-discovery pattern
- Update Quick Start with Run.cmd/Run.ps1 launcher instructions
- Add gen_packages.ps1 and -BuildLibs usage
- Update repository structure tree to match current layout
- Add CI/CD workflows reference
- Enhance contributing guidelines with ISample implementation tips

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: JusterZhu