docs: Improved DI documentation (#2407)

docs/guides/dependency_injection/samples/program.cs

@@ -0,0 +1,24 @@
+public class Program
+{
+    private readonly IServiceProvider _serviceProvider;
+
+    public Program()
+    {
+        _serviceProvider = CreateProvider();
+    }
+
+    static void Main(string[] args)
+        => new Program().RunAsync(args).GetAwaiter().GetResult();
+
+    static IServiceProvider CreateProvider()
+    {
+        var collection = new ServiceCollection();
+        //...
+        return collection.BuildServiceProvider();
+    }
+
+    async Task RunAsync(string[] args)
+    {
+        //...
+    }
+}