docs: Improved DI documentation (#2407)

docs/guides/dependency_injection/samples/collection.cs

@@ -0,0 +1,13 @@
+static IServiceProvider CreateServices()
+{
+    var config = new DiscordSocketConfig()
+    {
+        //...
+    };
+
+    var collection = new ServiceCollection()
+        .AddSingleton(config)
+        .AddSingleton<DiscordSocketClient>();
+
+    return collection.BuildServiceProvider();
+}