Give radio button its own example code now

Still within a RadioSet, but this time only the code that uses RadioSet; not
the code that just uses strings.

docs/examples/widgets/radio_button.css

@@ -0,0 +1,7 @@
+Screen {
+    align: center middle;
+}
+
+RadioSet {
+    width: 50%;
+}

docs/examples/widgets/radio_button.py

@@ -0,0 +1,26 @@
+from textual.app import App, ComposeResult
+from textual.widgets import RadioButton, RadioSet
+
+
+class RadioChoicesApp(App[None]):
+    CSS_PATH = "radio_button.css"
+
+    def compose(self) -> ComposeResult:
+        yield RadioSet(
+            RadioButton("Battlestar Galactica"),
+            RadioButton("Dune 1984"),
+            RadioButton("Dune 2021", id="focus_me"),
+            RadioButton("Serenity", value=True),
+            RadioButton("Star Trek: The Motion Picture"),
+            RadioButton("Star Wars: A New Hope"),
+            RadioButton("The Last Starfighter"),
+            RadioButton("Total Recall :backhand_index_pointing_right: :red_circle:"),
+            RadioButton("Wing Commander"),
+        )
+
+    def on_mount(self) -> None:
+        self.query_one("#focus_me", RadioButton).focus()
+
+
+if __name__ == "__main__":
+    RadioChoicesApp().run()