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%;
+}