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/MediatrConfiguringDI.cs

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+.AddMediatR(typeof(Bot))

docs/guides/other_libs/samples/MediatrCreatingMessageNotification.cs

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+// 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; }
+}

docs/guides/other_libs/samples/MediatrDiscordEventListener.cs

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+// DiscordEventListener.cs
+
+using Discord.WebSocket;
+using MediatR;
+using MediatRSample.Notifications;
+using Microsoft.Extensions.DependencyInjection;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MediatRSample;
+
+public class DiscordEventListener
+{
+    private readonly CancellationToken _cancellationToken;
+
+    private readonly DiscordSocketClient _client;
+    private readonly IServiceScopeFactory _serviceScope;
+
+    public DiscordEventListener(DiscordSocketClient client, IServiceScopeFactory serviceScope)
+    {
+        _client = client;
+        _serviceScope = serviceScope;
+        _cancellationToken = new CancellationTokenSource().Token;
+    }
+
+    private IMediator Mediator
+    {
+        get
+        {
+            var scope = _serviceScope.CreateScope();
+            return scope.ServiceProvider.GetRequiredService<IMediator>();
+        }
+    }
+
+    public async Task StartAsync()
+    {
+        _client.MessageReceived += OnMessageReceivedAsync;
+
+        await Task.CompletedTask;
+    }
+
+    private Task OnMessageReceivedAsync(SocketMessage arg)
+    {
+        return Mediator.Publish(new MessageReceivedNotification(arg), _cancellationToken);
+    }
+}

docs/guides/other_libs/samples/MediatrMessageReceivedHandler.cs

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+// 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
+    }
+}

docs/guides/other_libs/samples/MediatrStartListener.cs

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+// Program.cs
+
+var listener = services.GetRequiredService<DiscordEventListener>();
+await listener.StartAsync();