Content update - Dependency properties overview (user story 1878471) (#1160)

* Update article and code projects

* Further edits

* Fix links

* Relocate article & snippets to wpf/properties folder

* add toc and redirects

* Add acrolinx comment and make a minor edit.

* minor

Co-authored-by: Andy De George (adegeo) <[email protected]>

dotnet-desktop-guide/net/wpf/properties/snippets/dependency-properties-overview/csharp/App.xaml.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+using System.Configuration;
+using System.Data;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows;
+
+namespace CodeSampleCsharp
+{
+    /// <summary>
+    /// Interaction logic for App.xaml
+    /// </summary>
+    public partial class App : Application
+    {
+    }
+}