Mark IPageTopicViewModel as obsolete

There's no expected use case for the `IPageTopicViewModel`. We foresee the limited but potential need for e.g. shared view models that will handle navigation, but these can be handled via `INavigableTopicViewModel` and `INavigationTopicViewModel<T>`. Others _may_ need additional details, which are available via `ITopicViewModel`. But we don't expect reusable _layouts_, which is where the additional requirements of e.g. `IPageTopicViewModel` comes in.

On an assessment of OnTopic 4.x implementations, it doesn't appear anyone is utilizing `IPageTopicViewModel`, except possibly on model definitions, so it should be safe to remove. Marking it as obsolete.

Author: JeremyCaney

OnTopic/Models/IPageTopicViewModel.cs

@@ -3,6 +3,7 @@
 | Client        Ignia, LLC
 | Project       Topics Library
 \=============================================================================================================================*/
+using System;
 using OnTopic.Mapping;
 
 namespace OnTopic.Models {
@@ -21,6 +22,7 @@ namespace OnTopic.Models {
   ///     provided via the public interface then it will instead need to be defined in some other way.
   ///   </para>
   /// </remarks>
+  [Obsolete("The IPageTopicViewModel is no longer utilized.", true)]
   public interface IPageTopicViewModel : ITopicViewModel, INavigableTopicViewModel {
 
     /*==========================================================================================================================