Added XMLDoc documentation for NavigationViewModel<T>

The purpose of these properties may not be immediately clear—and especially in terms of how they relate to `INavigationTopicViewModel<T>`—so adding documentation helps clarify the relationships in the fairly complex landscope of the navigation view components. This also ensures IntelliSense support.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/Models/NavigationViewModel{T}.cs

@@ -27,7 +27,40 @@ namespace OnTopic.AspNetCore.Mvc.Models {
   /// </remarks>
   public class NavigationViewModel<T> where T : class, INavigationTopicViewModel<T> {
 
+    /*==========================================================================================================================
+    | NAVIGATION ROOT
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Represents the root element in the navigation.
+    /// </summary>
+    /// <remarks>
+    ///   Since this implements <see cref="IHierarchicalTopicViewModel{T}"/>, it  may include multiple levels of children. By
+    ///   implementing it as a generic, each site or application can provide its own <see cref="INavigationTopicViewModel{T}"/>
+    ///   implementation, thus potentially extending the schema with properties relevant to that site's navigation. For example,
+    ///   a site may optionally add an <c>IconUrl</c> property if it wishes to assign unique icons to each link in the
+    ///   navigation.
+    /// </remarks>
     public T? NavigationRoot { get; set; }
+
+    /*==========================================================================================================================
+    | CURRENT KEY
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   The <see cref="Topic.GetUniqueKey()"/> representing the path to the current <see cref="Topic"/>.
+    /// </summary>
+    /// <remarks>
+    ///   <para>
+    ///     In order to determine whether any given <see cref="INavigationTopicViewModel{T}.IsSelected(string)"/>, the views
+    ///     will need to know where in the hierarchy the user currently is. By storing this on the <see
+    ///     cref="NavigationViewModel{T}"/> used as the root view model for every navigation component, we ensure that the views
+    ///     always have access to this information.
+    ///   </para>
+    ///   <para>
+    ///     It's worth noting that while this <i>could</i> be stored on the <see cref="INavigationTopicViewModel{T}"/> itself,
+    ///     that would prevent those values from being cached. As such, it's preferrable to keep the navigation nodes stateless,
+    ///     and maintaining state exclusively in the <see cref="NavigationViewModel{T}"/> itself.
+    ///   </para>
+    /// </remarks>
     public string CurrentKey { get; set; } = default!;
 
   } //Class