docs(ci): add comprehensive guide for OpenMaui Linux samples
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.
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