Doc: Fix linking to 'Drag and Drop' topic

Linking to 'Drag and Drop' target will link to a page local to Qt
Widgets with that section title. The intention is to link to the
topic page located in Qt GUI module.

Add a descriptive keyword to the correct target page and use that
for linking.

Fixes: QTBUG-109289
Change-Id: Ib6a39b43f9080cf5bb72fb07ab163e75eefb4da9
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
(cherry picked from commit 8a30b22930d3a3d15fba97ff7aec7f812721869e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/doc/src/dnd.qdoc

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
 /*!
+    \keyword Drag and Drop in Qt
     \page dnd.html
     \title Drag and Drop
     \brief An overview of the drag and drop system provided by Qt.

src/widgets/kernel/qapplication.cpp

@@ -2476,7 +2476,7 @@ void QApplication::setStartDragTime(int ms)
 
     The default value is 500 ms.
 
-    \sa startDragDistance(), {Drag and Drop}
+    \sa startDragDistance(), {Drag and Drop in Qt}{Drag and Drop}
 */
 
 int QApplication::startDragTime()
@@ -2515,7 +2515,7 @@ void QApplication::setStartDragDistance(int l)
     The default value (if the platform doesn't provide a different default)
     is 10 pixels.
 
-    \sa startDragTime(), QPoint::manhattanLength(), {Drag and Drop}
+    \sa startDragTime(), QPoint::manhattanLength(), {Drag and Drop in Qt}{Drag and Drop}
 */
 
 int QApplication::startDragDistance()

src/widgets/kernel/qwidget.cpp

@@ -515,7 +515,7 @@ void QWidget::setAutoFillBackground(bool enabled)
             button is held down. This can be useful during drag and drop
             operations. If you call \l{setMouseTracking()}{setMouseTracking}(true),
             you get mouse move events even when no buttons are held down.
-            (See also the \l{Drag and Drop} guide.)
+            (See also the \l{Drag and Drop in Qt}{Drag and Drop} guide.)
         \li  keyReleaseEvent() is called whenever a key is released and while it
             is held down (if the key is auto-repeating). In that case, the
             widget will receive a pair of key release and key press event for
@@ -3431,7 +3431,7 @@ void QWidgetPrivate::setEnabled_helper(bool enable)
 
     By default, this property is \c false.
 
-    \sa {Drag and Drop}
+    \sa {Drag and Drop in Qt}{Drag and Drop}
 */
 bool QWidget::acceptDrops() const
 {