docs: Improved DI documentation (#2407)

docs/guides/dependency_injection/samples/service-registration.cs

@@ -0,0 +1,21 @@
+static IServiceProvider CreateServices()
+{
+    var config = new DiscordSocketConfig()
+    {
+        //...
+    };
+
+    // X represents either Interaction or Command, as it functions the exact same for both types.
+    var servConfig = new XServiceConfig()
+    {
+        //...
+    }
+
+    var collection = new ServiceCollection()
+        .AddSingleton(config)
+        .AddSingleton<DiscordSocketClient>()
+        .AddSingleton(servConfig)
+        .AddSingleton<XService>();
+
+    return collection.BuildServiceProvider();
+}