more docs and diagrams

docs/examples/guide/dom2.py

@@ -0,0 +1,11 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Header, Footer
+
+
+class ExampleApp(App):
+    def compose(self) -> ComposeResult:
+        yield Header()
+        yield Footer()
+
+
+app = ExampleApp()