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/MediatrMessageReceivedHandler.cs

@@ -0,0 +1,17 @@
+// MessageReceivedHandler.cs
+
+using System;
+using MediatR;
+using MediatRSample.Notifications;
+
+namespace MediatRSample;
+
+public class MessageReceivedHandler : INotificationHandler<MessageReceivedNotification>
+{
+    public async Task Handle(MessageReceivedNotification notification, CancellationToken cancellationToken)
+    {
+        Console.WriteLine($"MediatR works! (Received a message by {notification.Message.Author.Username})");
+
+        // Your implementation
+    }
+}