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/toc.yml

@@ -94,6 +94,8 @@ items:
       href: properties/dependency-property-metadata.md
     - name: Dependency property callbacks and validation
       href: properties/dependency-property-callbacks-and-validation.md
+    - name: Read-only dependency properties
+      href: properties/read-only-dependency-properties.md
     - name: Common tasks
       items:
       - name: Implement a dependency property