app basics and doc structure

docs/examples/app/widgets01.py

@@ -0,0 +1,15 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Welcome
+
+
+class WelcomeApp(App):
+    def compose(self) -> ComposeResult:
+        yield Welcome()
+
+    def on_button_pressed(self) -> None:
+        self.exit()
+
+
+app = WelcomeApp()
+if __name__ == "__main__":
+    app.run()