Tell all concrete implementations to be private / not show in the docs

Oqtane.Client/Services/FolderService.cs

@@ -7,9 +7,11 @@ using Oqtane.Shared;
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Net;
+using Oqtane.Documentation;
 
 namespace Oqtane.Services
 {
+    [PrivateApi("Don't show in the documentation, as everything should use the Interface")]
     public class FolderService : ServiceBase, IFolderService
     {
         private readonly SiteState _siteState;