Add width reference documentation for radio buttons

A large part of the code to go with this is to show off a radio set; I feel
it makes sense to use the same code for both bits of documentation given
that a radio button only really makes sense inside a radio set.

docs/examples/widgets/radio_set.css

@@ -0,0 +1,12 @@
+Screen {
+    align: center middle;
+}
+
+Horizontal {
+    align: center middle;
+    height: auto;
+}
+
+RadioSet {
+    width: 45%;
+}

docs/examples/widgets/radio_set.py

@@ -0,0 +1,43 @@
+from textual.app import App, ComposeResult
+from textual.containers import Horizontal
+from textual.widgets import RadioButton, RadioSet
+
+
+class RadioChoicesApp(App[None]):
+    CSS_PATH = "radio_set.css"
+
+    def compose(self) -> ComposeResult:
+        yield Horizontal(
+            RadioSet(
+                RadioButton("Battlestar Galactica"),
+                RadioButton("Dune 1984"),
+                RadioButton("Dune 2021"),
+                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:",
+                    id="focus_me",
+                ),
+                RadioButton("Wing Commander"),
+            ),
+            RadioSet(
+                "Amanda",
+                "Connor MacLeod",
+                "Duncan MacLeod",
+                "Heather MacLeod",
+                "Joe Dawson",
+                "Kurgan, [bold italic red]The[/]",
+                "Methos",
+                "Rachel Ellenstein",
+                "Ramírez",
+            ),
+        )
+
+    def on_mount(self) -> None:
+        self.query_one("#focus_me", RadioButton).focus()
+
+
+if __name__ == "__main__":
+    RadioChoicesApp().run()