Add missing BindableAttribute information for C++/WinRT data binding in Windows App SDK documentation

[Copilot]

The Windows App SDK version of the "Data Binding In Depth" article was missing critical information about when BindableAttribute is required for C++/WinRT when using the {Binding} markup extension. This information was present in the UWP version of the same article but was inadvertently lost during the transition to Windows App SDK documentation.

Changes Made

Added BindableAttribute requirement information in two key sections:

1. Binding target section - Added note about BindableAttribute requirement
2. Binding object declared using {Binding} section - Added comprehensive information including:
   • When BindableAttribute is required for C++/WinRT
   • C++/WinRT code example showing proper attribute usage
   • Important note about Windows SDK version requirements
   • Fallback information for older SDK versions

Example of added C++/WinRT code:

// HostViewModel.idl
// Add this attribute:
[Microsoft.UI.Xaml.Data.Bindable]
runtimeclass HostViewModel : Microsoft.UI.Xaml.Data.INotifyPropertyChanged
{
    HostViewModel();
    String NextButtonText;
}

Key information restored:

• BindableAttribute is required for C++/WinRT runtime classes when using {Binding} markup extension
• Attribute is available starting with Windows SDK version 10.0.17763.0 (Windows 10, version 1809)
• Fallback to implementing ICustomPropertyProvider and ICustomProperty interfaces when BindableAttribute is not available
• No attribute needed when using {x:Bind} markup extension

All API references have been updated to use the appropriate Windows App SDK namespaces (Microsoft.UI.Xaml.Data instead of Windows.UI.Xaml.Data).

Fixes #5037.

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