Link the Header docs to App.title and App.sub_title

Closes #3103.

docs/examples/widgets/header_app_title.py

@@ -0,0 +1,16 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Header
+
+
+class HeaderApp(App):
+    def compose(self) -> ComposeResult:
+        yield Header()
+
+    def on_mount(self) -> None:
+        self.title = "Header Application"
+        self.sub_title = "With title and sub-title"
+
+
+if __name__ == "__main__":
+    app = HeaderApp()
+    app.run()