Document setting app (sub)title.

docs/examples/app/question_title01.py

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+from textual.app import App, ComposeResult
+from textual.widgets import Button, Header, Label
+
+
+class MyApp(App[str]):
+    TITLE = "A Question App"
+    SUB_TITLE = "The most important question"
+
+    def compose(self) -> ComposeResult:
+        yield Header()
+        yield Label("Do you love Textual?")
+        yield Button("Yes", id="yes", variant="primary")
+        yield Button("No", id="no", variant="error")
+
+    def on_button_pressed(self, event: Button.Pressed) -> None:
+        self.exit(event.button.id)
+
+
+if __name__ == "__main__":
+    app = MyApp()
+    reply = app.run()
+    print(reply)