Remove Xamarin platform documentation and update TOC (#919)

Deleted legacy Xamarin-related installation and guidelines documentation, including Xamarin.Android, Xamarin.Forms, Xamarin.iOS, and Xamarin.Mac. Updated the installation table of contents to remove Xamarin entries and add AndroidX. Also removed references to legacy Fody and Xamarin packages from the project structure documentation.

Author: ChrisPulman

reactiveui/contribute/compiling-reactive-ui/project-structure.md

@@ -20,15 +20,3 @@ The [ReactiveUI.WinUI](~/api/reactiveui.winui.yml) package contains the Reactive
 
 ## ReactiveUI.Wpf
 The [ReactiveUI.Wpf](~/api/reactiveui.wpf.yml) package contains the ReactiveUI platform-specific extensions for [Windows Presentation Foundation (WPF)](https://learn.microsoft.com/en-us/dotnet/desktop/wpf/overview/).
-
-## ReactiveUI.Fody (legacy)
-The [ReactiveUI.Fody](~/api/ReactiveUI.Fody.yml) package is a [Fody](https://github.com/Fody/Fody) extension that generates property change notifications and ObservableAsPropertyHelpers. It remains available but is no longer the recommended approach for new projects.
-
-## ReactiveUI.Fody.Analyzer (legacy)
-The [ReactiveUI.Fody.Analyzer](~/api/ReactiveUI.Fody.Analyzer.yml) package is a [Roslyn](https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/) analyzer that checks usage of the Fody extension.
-
-## ReactiveUI.Fody.Helpers (legacy)
-The [ReactiveUI.Fody.Helpers](~/api/ReactiveUI.Fody.Helpers.yml) package extends Fody helpers. Prefer SourceGenerators going forward.
-
-## ReactiveUI.XamForms (legacy)
-The [ReactiveUI.XamForms](~/api/reactiveui.xamforms.yml) package contains the ReactiveUI platform-specific extensions for Xamarin.Forms. Xamarin platforms are legacy and supported for maintenance only.