Update XamlCompositionBrushBase examples for Windows App SDK

[Copilot]

The Microsoft.UI.Xaml.Media.XamlCompositionBrushBase docs still showed UWP-era examples, including Window.Current.Compositor, which is not the right pattern for Windows App SDK/WinUI 3. This updates the samples and links to align with the current API surface.

• Example code updates

  • Replaced Window.Current.Compositor with CompositionTarget.GetCompositorForCurrentThread()
  • Updated sample type references from Windows.UI.Xaml.* / Windows::UI::* to Microsoft.UI.Xaml.* / Microsoft::UI::*
  • Switched composition types in samples to the Microsoft.UI.Composition equivalents

• Doc link cleanup

  • Repointed UWP composition/brush links to current Windows apps / Windows App SDK documentation
  • Updated inline API links to the corresponding Microsoft.UI.* reference pages in this repo

• Sample correctness

  • Corrected the C++/CX BlurAmount dependency property registration to use double::typeid

protected override void OnConnected()
{
    if (CompositionBrush == null)
    {
        var compositor = CompositionTarget.GetCompositorForCurrentThread();
        var backdrop = compositor.CreateBackdropBrush();

        var effectFactory = compositor.CreateEffectFactory(
            graphicsEffect,
            new[] { "Blur.BlurAmount" });

        var effectBrush = effectFactory.CreateBrush();
        effectBrush.SetSourceParameter("backdrop", backdrop);

        CompositionBrush = effectBrush;
    }
}

• Fixes Wrong examples in Microsoft.UI.Xaml.Media.XamlCompositionBrushBase #106