Use ZIndex in the Arena

Author: lisandroct

docs/docs/00100-intro/00300-tutorials/00500-godot-tutorial/00400-part-3.md

@@ -971,7 +971,7 @@ To render both Circle and Food entities we need a way to draw circles on the scr
 ```csharp
 using Godot;
 
-public partial class Circle2D : Node2D
+public abstract partial class Circle2D : Node2D
 {
     private float _radius = 10.0f;
     [Export]
@@ -1584,7 +1584,8 @@ private void SetupArena(float worldSize)
         Name = "Background",
         Color = BackgroundColor,
         Position = Vector2.Zero,
-        Polygon = polygon
+        Polygon = polygon,
+        ZIndex = -1000
     };
     background.AddChild(new Polygon2D
     {
@@ -1595,13 +1596,13 @@ private void SetupArena(float worldSize)
         InvertBorder = BorderThickness,
         Polygon = polygon
     });
-    AddChild(background, @internal: InternalMode.Front);
+    AddChild(background);
 
     AddChild(new CameraController(worldSize));
 }
 ```
 
-Note that we added a new parameter to the first `AddChild` where we add the `background`: `@internal: InternalMode.Front`. This is to tell Godot to add this child (the background) before the other siblings, so it goes behind the `Instantiator` that we added earlier.
+Note that we defined a negative `ZIndex` to `background`. This is so it gets render behind all the other siblings (instantiated by the `Instantiator` that we added earlier).
 
 ### Entering the Game