Add 1200+ line GUIDE.md documenting OpenMaui Linux sample applications. Covers prerequisites, installation, building, and running three sample apps (ShellDemo, TodoApp, WebViewDemo).
Includes detailed sections on:
- Creating new Linux MAUI applications
- Adding OpenMaui Linux support to existing projects
- Implementing navigation patterns (Shell and NavigationPage)
- Working with themes and styling
- API reference for MauiProgram, platform entry points, and dialogs
- Troubleshooting common issues
Update README.md with improved overview and links to user guide.
