Doc: Add missing CheckDelegate control and ingroup ItemDelegate

* Add missing ingroup command to ItemDelegate to be listed by
  annotatedlist command on qtquickcontrols2-delegates page

* Add missing image and description for CheckDelegate
  Control on qtquickcontrols2-delegates page

Task-number: QTBUG-104157
Pick-to: 6.2 6.3 6.4
Change-Id: I0b8d830aeab348c763d4e716d5962258c9c82a77
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

src/quickcontrols2/doc/src/qtquickcontrols2-delegates.qdoc

@@ -16,6 +16,16 @@
     sections offer guidelines for choosing the appropriate type of delegate,
     depending on the use case.
 
+    \section1 CheckDelegate Control
+
+    \image qtquickcontrols2-checkdelegate.gif
+
+    \l CheckDelegate presents a checkable control that can be toggled on
+    (checked) or off (unchecked). Check delegates are typically used to
+    select one or more options from a set of options.
+
+    \b {See also} \l {CheckBox Control}.
+
     \section1 ItemDelegate Control
 
     \image qtquickcontrols2-itemdelegate.gif

src/quicktemplates2/qquickitemdelegate.cpp

@@ -15,6 +15,7 @@ QT_BEGIN_NAMESPACE
 //!     \instantiates QQuickItemDelegate
     \inqmlmodule QtQuick.Controls
     \since 5.7
+    \ingroup qtquickcontrols2-delegates
     \brief Basic item delegate that can be used in various views and controls.
 
     \image qtquickcontrols2-itemdelegate.gif