Content update - Read-only dependency properties (user story 1878471) (#1230)

* Improve collection-type dp snippets that are readonly.

* Add article, snippets, toc, and redirects

* Minor edit

* Update target framework to .NET 6.0

dotnet-desktop-guide/net/wpf/properties/snippets/read-only-dependency-properties/vb/MainWindow.xaml

@@ -0,0 +1,12 @@
+<Window x:Class="CodeSampleVb.MainWindow"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="Readonly dependency properties" Height="100" Width="400">
+
+    <StackPanel>
+        
+        <Button Click="Button_Click">Increment property value</Button>
+        <Label Name="lblFishCount" Content="Aquarium fish count: 0"/>
+
+    </StackPanel>
+</Window>