Revamp installation docs for all major platforms (#923)

Expanded and modernized the installation and getting started guides for Avalonia, Blazor, MAUI, Windows Forms, and WPF. Updated all guides to use RxAppBuilder and SourceGenerators, provided detailed code samples, and clarified recommended project structures and dependency injection patterns. Updated package references and badges in the index. Improved clarity, consistency, and completeness across all platform documentation.

Author: ChrisPulman