Fix Narrator announcing 'SL NO' instead of serial number on What's New page

The 'Sl. No.' column header in the Grid Shorthand Syntax sample on the What's New page was read literally by Narrator (as 'SL NO') instead of as a serial number. Add AutomationProperties.Name="Serial Number" to the header cell so screen readers announce a meaningful name. Addresses MAS 4.1.2 (Name, Role, Value). Fixes AB#3019248.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mmanolova-msft

Sample Applications/WPFGallery/ViewModels/WhatsNewPageViewModel.cs

@@ -69,7 +69,7 @@ public void Navigate(object pageType)
 
         private const string _gridShorthandSyntaxXamlUsage = """
             <Grid RowDefinitions="Auto,Auto,Auto" ColumnDefinitions="Auto 80 *" HorizontalAlignment="Left">
-                <TextBlock Grid.Row="0" Grid.Column="0" FontWeight="Bold" Margin="0 0 10 0">Sl. No.</TextBlock>
+                <TextBlock Grid.Row="0" Grid.Column="0" FontWeight="Bold" Margin="0 0 10 0" AutomationProperties.Name="Serial Number">Sl. No.</TextBlock>
                 <TextBlock Grid.Row="0" Grid.Column="1" FontWeight="Bold">Name</TextBlock>
                 <TextBlock Grid.Row="0" Grid.Column="2" FontWeight="Bold">Description</TextBlock>
                 <TextBlock Grid.Row="1" Grid.Column="0">1</TextBlock>

Sample Applications/WPFGallery/Views/WhatsNewPage.xaml

@@ -74,7 +74,7 @@
                     HeaderText="Grid Shorthand Syntax Sample"
                     XamlCode="{Binding ViewModel.GridShorthandSyntaxXamlCode}">
                         <Grid RowDefinitions="Auto,Auto,Auto" ColumnDefinitions="Auto 80 *" HorizontalAlignment="Left">
-                            <TextBlock Grid.Row="0" Grid.Column="0" FontWeight="Bold" Margin="0 0 10 0">Sl. No.</TextBlock>
+                            <TextBlock Grid.Row="0" Grid.Column="0" FontWeight="Bold" Margin="0 0 10 0" AutomationProperties.Name="Serial Number">Sl. No.</TextBlock>
                             <TextBlock Grid.Row="0" Grid.Column="1" FontWeight="Bold">Name</TextBlock>
                             <TextBlock Grid.Row="0" Grid.Column="2" FontWeight="Bold">Description</TextBlock>
                             <TextBlock Grid.Row="1" Grid.Column="0">1</TextBlock>