reorganize RyujinxSetupWizard

additionally, the CreateHelpContent method is now no longer locked to returning an Avalonia `Control`.
the WithHelpContent method also has logic to handle it returning a string directly, and will wrap it in a textblock with h1 style and size 20 font. otherwise it's up to the ContentPresenter for the help content to choose how to display it if it's none of the above mentioned types.

src/Ryujinx/UI/SetupWizard/SetupWizardPageContext.cs

@@ -1,4 +1,3 @@
-using Avalonia.Controls;
 using Gommon;
 using Ryujinx.Ava.UI.Helpers;
 using Ryujinx.Ava.UI.ViewModels;
@@ -10,8 +9,9 @@ namespace Ryujinx.Ava.UI.SetupWizard
         public RyujinxNotificationManager NotificationManager { get; init; }
 
         public abstract Result CompleteStep();
-
-        public virtual Control CreateHelpContent()
+#nullable enable
+        public virtual object? CreateHelpContent()
+#nullable disable
         {
             return null;
         }