Add an example for creating a button in the Button class documentation

This closes https://github.com/godotengine/godot-docs/issues/3798.

doc/classes/Button.xml

@@ -5,6 +5,18 @@
 	</brief_description>
 	<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]
+		func _ready():
+		    var button = Button.new()
+		    button.text = "Click me"
+		    button.connect("pressed", self, "_button_pressed")
+		    add_child(button)
+
+		func _button_pressed():
+		    print("Hello world!")
+		[/codeblock]
+		Buttons (like all Control nodes) can also be created in the editor, but some situations may require creating them from code.
 	</description>
 	<tutorials>
 	</tutorials>