Overhaul setup wizard help pages

the context can now override a virtual method named `CreateHelpContent` which the setup wizard system will automatically try to use when you use the generic overload taking a generic context type. If the return is null, it skips setting entirely (the default impl is null)

additionally made the discord join link a button with code copied from the about window, and made it centered at the bottom.

src/Ryujinx/UI/SetupWizard/SetupWizardPageContext.cs

@@ -1,3 +1,4 @@
+using Avalonia.Controls;
 using Gommon;
 using Ryujinx.Ava.UI.Helpers;
 using Ryujinx.Ava.UI.ViewModels;
@@ -9,5 +10,10 @@ namespace Ryujinx.Ava.UI.SetupWizard
         public RyujinxNotificationManager NotificationManager { get; init; }
 
         public abstract Result CompleteStep();
+
+        public virtual Control CreateHelpContent()
+        {
+            return null;
+        }
     }
 }