docs: Improved DI documentation (#2407)

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

@@ -0,0 +1,12 @@
+public static ServiceCollection RegisterImplicitServices(this ServiceCollection collection, Type interfaceType, Type activatorType)
+{
+    // Get all types in the executing assembly. There are many ways to do this, but this is fastest.
+    foreach (var type in typeof(Program).Assembly.GetTypes())
+    {
+        if (interfaceType.IsAssignableFrom(type) && !type.IsAbstract)
+            collection.AddSingleton(interfaceType, type);
+    }
+
+    // Register the activator so you can activate the instances.
+    collection.AddSingleton(activatorType);
+}