actions docs

docs/examples/guide/actions/actions01.py

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+from textual.app import App
+from textual import events
+
+
+class ActionsApp(App):
+    def action_set_background(self, color: str) -> None:
+        self.screen.styles.background = color
+        self.bell()
+
+    def on_key(self, event: events.Key) -> None:
+        if event.key == "r":
+            self.action_set_background("red")
+
+
+if __name__ == "__main__":
+    app = ActionsApp()
+    app.run()