MediatR Guide + sample (#2218)

* Add guide for MediatR

* Add sample for MediatR

* Fix exposed token in program.cs

* Fix review points

* Remove newline in MediatrDiscordEventListener.cs

docs/guides/other_libs/samples/MediatrCreatingMessageNotification.cs

@@ -0,0 +1,16 @@
+// MessageReceivedNotification.cs
+
+using Discord.WebSocket;
+using MediatR;
+
+namespace MediatRSample.Notifications;
+
+public class MessageReceivedNotification : INotification
+{
+    public MessageReceivedNotification(SocketMessage message)
+    {
+        Message = message ?? throw new ArgumentNullException(nameof(message));
+    }
+
+    public SocketMessage Message { get; }
+}