add a few more links to WinUI Gallery (#5625)

Author: jwmsft

hub/apps/develop/ui-input/manage-app-windows.md

@@ -194,6 +194,9 @@ The window should have an owner when IsModal=true.
 
 To set the owner window in a WinUI app requires Win32 interop. For more information and example code, see the AppWindow page in the WinUI Gallery sample app.
 
+- [Launch the WinUI Gallery app](winui3gallery://item/AppWindow)
+- [Open ModalWindow.xaml.cs on GitHub](https://github.com/microsoft/WinUI-Gallery/blob/main/WinUIGallery/Samples/SamplePages/ModalWindow.xaml.cs)
+
 ### Apply a presenter
 
 A presenter can be applied to only a single window at a time. Trying to apply the same presenter to a second window throws an exception. That means that if you have multiple windows, and you want to switch each one into a specific presentation mode, then you need to create multiple presenters of the same kind, and then apply each to its own window.

hub/apps/develop/ui-input/windowing-overview.md

@@ -22,6 +22,11 @@ Windowing functionality in a WinUI app is provided by a combination of the XAML
 >
 > - **Important APIs**: [Window class](/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.window), [AppWindow class](/windows/windows-app-sdk/api/winrt/microsoft.ui.windowing.appwindow)
 
+> [!div class="nextstepaction"]
+> [Open the WinUI 3 Gallery app and see Windowing samples in action](winui3gallery://category/MultipleWindows)
+
+[!INCLUDE [winui-3-gallery](../../../includes/winui-3-gallery.md)]
+
 **XAML Window**
 
 In your app, the window object is an instance of the [Microsoft.UI.Xaml.Window](/windows/windows-app-sdk/api/winrt/microsoft.ui.xaml.window) class (or a derived class) that represents the window in your program code. You create it directly with a call to the constructor. The XAML Window is where you attach your app content and manage the lifecycle of your app's windows.