Add the widget reference for Checkbox

docs/examples/widgets/checkbox.css

@@ -0,0 +1,11 @@
+Screen {
+    align: center middle;
+}
+
+Vertical {
+    width: auto;
+    height: auto;
+    border: solid $boost;
+    background: $panel;
+    padding: 2;
+}

docs/examples/widgets/checkbox.py

@@ -0,0 +1,26 @@
+from textual.app import App, ComposeResult
+from textual.containers import Vertical
+from textual.widgets import Checkbox
+
+
+class CheckboxApp(App[None]):
+    CSS_PATH = "checkbox.css"
+
+    def compose(self) -> ComposeResult:
+        yield Vertical(
+            Checkbox("Arrakis :sweat:"),
+            Checkbox("Caladan"),
+            Checkbox("Chusuk"),
+            Checkbox("[b]Giedi Prime[/b]"),
+            Checkbox("[magenta]Ginaz[/]"),
+            Checkbox("Grumman", True),
+            Checkbox("Kaitain", id="initial_focus"),
+            Checkbox("Novebruns", True),
+        )
+
+    def on_mount(self):
+        self.query_one("#initial_focus", Checkbox).focus()
+
+
+if __name__ == "__main__":
+    CheckboxApp().run()