Add C# code examples to the docs

Only existing GDScript code examples are converted and added to the
docs.
This is the first batch include classes beginning with A and B.

Included classes:
 * AcceptDialog
 * AESContext
 * Animation
 * AnimationNodeStateMachine
 * AnimationNodeStateMachinePlayback
 * AnimationNodeStateMachineTransition
 * Array
 * ArrayMesh
 * AStar
 * AStar2D
 * Bool
 * Button

doc/classes/Button.xml

@@ -6,7 +6,8 @@
 	<description>
 		Button is the standard themed button. It can contain text and an icon, and will display them according to the current [Theme].
 		[b]Example of creating a button and assigning an action when pressed by code:[/b]
-		[codeblock]
+		[codeblocks]
+		[gdscript]
 		func _ready():
 		    var button = Button.new()
 		    button.text = "Click me"
@@ -15,7 +16,22 @@
 
 		func _button_pressed():
 		    print("Hello world!")
-		[/codeblock]
+		[/gdscript]
+		[csharp]
+		public override void _Ready()
+		{
+		    var button = new Button();
+		    button.Text = "Click me";
+		    button.Connect("pressed", this, nameof(ButtonPressed));
+		    AddChild(button);
+		}
+
+		private void ButtonPressed()
+		{
+		    GD.Print("Hello world!");
+		}
+		[/csharp]
+		[/codeblocks]
 		Buttons (like all Control nodes) can also be created in the editor, but some situations may require creating them from code.
 		See also [BaseButton] which contains common properties and methods associated with this node.
 	</description>