app basics and doc structure

docs/examples/app/return.py

@@ -0,0 +1,14 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Button
+
+
+class ButtonsApp(App):
+    def compose(self) -> ComposeResult:
+        yield Button("Paul")
+        yield Button("Duncan")
+        yield Button("Chani")
+
+
+app = ButtonsApp()
+if __name__ == "__main__":
+    app.run()