more docs and diagrams

docs/examples/guide/dom1.py

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+from textual.app import App
+
+
+class ExampleApp(App):
+    pass
+
+
+app = ExampleApp()

docs/examples/guide/dom2.py

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+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()

docs/examples/guide/dom3.py

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+from textual.app import App, ComposeResult
+from textual.layout import Container
+from textual.widgets import Header, Footer, Static
+
+
+class ExampleApp(App):
+    def compose(self) -> ComposeResult:
+        yield Header()
+        yield Footer()
+        yield Container(
+            Static(id="widget1"),
+            Static(id="widget2"),
+            Static(id="widget3"),
+        )
+
+
+app = ExampleApp()

docs/examples/guide/dom4.py

@@ -0,0 +1,18 @@
+from textual.app import App, ComposeResult
+from textual.layout import Container
+from textual.widget import Widget
+from textual.widgets import Header, Footer
+
+
+class ExampleApp(App):
+    def compose(self) -> ComposeResult:
+        yield Header()
+        yield Footer()
+        yield Container(
+            Widget(id="widget1"),
+            Widget(id="widget2"),
+            Widget(id="widget3"),
+        )
+
+
+app = ExampleApp()