Content update - XAML loading and dependency properties (user story 1878471) (#1257)

* Add article, toc, and redirects

* Add info on use of a callback to run custom logic and resolve merge conflicts

dotnet-desktop-guide/net/wpf/toc.yml

@@ -102,6 +102,8 @@ items:
       href: properties/dependency-property-security.md
     - name: Safe constructor patterns for DependencyObjects
       href: properties/safe-constructor-patterns-for-dependencyobjects.md
+    - name: XAML loading and dependency properties
+      href: properties/xaml-loading-and-dependency-properties.md
     - name: Common tasks
       items:
       - name: Implement a dependency property