Add the basics of Label docs for the manual

docs/examples/widgets/label.py

@@ -0,0 +1,12 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Label
+
+
+class LabelApp(App):
+    def compose(self) -> ComposeResult:
+        yield Label("Hello, world!")
+
+
+if __name__ == "__main__":
+    app = LabelApp()
+    app.run()