Add documentation for the widget gallery

Task-number: PYSIDE-1112
Change-Id: I35fe85d8ac9c2dab9f1260d3736c7f65e8e45d8e
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

examples/widgets/doc/src/gallery.qdoc

@@ -0,0 +1,36 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
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+** GNU Free Documentation License Usage
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+****************************************************************************/
+
+/*!
+    \example widgets/gallery
+    \title Widgets Gallery Example
+    \ingroup examples-widgets
+    \brief The Widgets Gallery example shows widgets relevant for designing UIs.
+
+    This example demonstrates widgets typically used in dialogs and forms.
+    It also allows for changing the style.
+*/