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/widgets/radiobutton.md

@@ -0,0 +1,59 @@
+# RadioButton
+
+A simple radio button which stores a boolean value.
+
+- [x] Focusable
+- [ ] Container
+
+## Example
+
+The example below shows radio buttons, used within [RadioSet][textual.widgets.RadioSet]s.
+
+=== "Output"
+
+    ```{.textual path="docs/examples/widgets/radio_set.py"}
+    ```
+
+=== "radio_set.py"
+
+    ```python
+    --8<-- "docs/examples/widgets/radio_set.py"
+    ```
+
+=== "radio_set.css"
+
+    ```sass
+    --8<-- "docs/examples/widgets/radio_set.css"
+    ```
+
+## Reactive Attributes
+
+| Name    | Type   | Default | Description                    |
+|---------|--------|---------|--------------------------------|
+| `value` | `bool` | `False` | The value of the radio button. |
+
+## Bindings
+
+The radio button widget defines directly the following bindings:
+
+::: textual.widgets._toggle_button.ToggleButton.BINDINGS
+    options:
+      show_root_heading: false
+      show_root_toc_entry: false
+
+## Component Classes
+
+The radio button widget provides the following component classes:
+
+::: textual.widgets._toggle_button.ToggleButton.COMPONENT_CLASSES
+    options:
+      show_root_heading: false
+      show_root_toc_entry: false
+
+## Messages
+
+### ::: textual.widgets.RadioButton.Changed
+
+## See Also
+
+- [RadioButton](../api/radiobutton.md) code reference