doc: Separate groupbox docs into a file of its own

That was probably an oversight

docs/reference/src/language/widgets/gridbox.md

@@ -3,29 +3,3 @@
 
 A `GridBox` is a [`GridLayout`](../builtins/elements.md#gridlayout) where the spacing and padding values
 depend on the style instead of defaulting to 0.
-
-## `GroupBox`
-
-A `GroupBox` is a container that groups its children together under a common title.
-
-### Properties
-
--   **`enabled`**: (_in_ _bool_): Defaults to true. When false, the groupbox can't be interacted with
--   **`title`** (_in_ _string_): A text written as the title of the group box.
-
-### Example
-
-```slint
-import { GroupBox , VerticalBox, CheckBox } from "std-widgets.slint";
-export component Example inherits Window {
-    width: 200px;
-    height: 100px;
-    GroupBox {
-        title: "Groceries";
-        VerticalLayout {
-            CheckBox { text: "Bread"; checked: true ;}
-            CheckBox { text: "Fruits"; }
-        }
-    }
-}
-```

docs/reference/src/language/widgets/groupbox.md

@@ -0,0 +1,27 @@
+<!-- Copyright © SixtyFPS GmbH <info@slint.dev> ; SPDX-License-Identifier: MIT -->
+
+## `GroupBox`
+
+A `GroupBox` is a container that groups its children together under a common title.
+
+### Properties
+
+-   **`enabled`**: (_in_ _bool_): Defaults to true. When false, the groupbox can't be interacted with
+-   **`title`** (_in_ _string_): A text written as the title of the group box.
+
+### Example
+
+```slint
+import { GroupBox , VerticalBox, CheckBox } from "std-widgets.slint";
+export component Example inherits Window {
+    width: 200px;
+    height: 100px;
+    GroupBox {
+        title: "Groceries";
+        VerticalLayout {
+            CheckBox { text: "Bread"; checked: true ;}
+            CheckBox { text: "Fruits"; }
+        }
+    }
+}
+```

docs/reference/src/language/widgets/index.rst

@@ -17,6 +17,7 @@ Widgets
    checkbox.md
    combobox.md
    gridbox.md
+   groupbox.md
    horizontalbox.md
    lineedit.md
    listview.md