Doc: Describe enabling the Qt Quick Designer plugin

The plugin has been disabled by default in 6.0, and
some users have already asked us about it.

Task-number: QTCREATORBUG-26278
Change-Id: I6085b17b0e78592eff285d2d0fc55107593202da
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

doc/qtcreator/src/qtcreator-toc.qdoc

@@ -102,6 +102,7 @@
             \li \l{Developing Qt Quick Applications}
             \list
                 \li \l {Creating Qt Quick Projects}
+                \li \l {Using \QMLD}
                 \li \l {Converting UI Projects to Applications}
                 \li \l {UI Files}
                 \li \l {Using QML Modules with Plugins}

doc/qtcreator/src/qtquick/creator-only/qtquick-app-development.qdoc

@@ -61,6 +61,11 @@
 
             You can use wizards to create Qt Quick projects.
 
+        \li \l {Using \QMLD}
+
+            You can enable the \QMLD plugin to visually edit
+            \l{UI Files}{UI files} (.ui.qml).
+
         \li \l {Converting UI Projects to Applications}
 
             Qt Quick UI projects (.qmlproject) are useful for creating user
@@ -73,7 +78,7 @@
             If you switch between \QC and \QDS or cooperate with designers on
             a project, you might encounter .ui.qml files. They are intended to
             be edited in \QDS only, so you need to be careful not to break the
-            code.
+            code. To visually edit the files in \QC, enable the \QMLD plugin.
 
         \li \l{Using QML Modules with Plugins}
 

doc/qtcreator/src/qtquick/creator-only/qtquick-creating.qdoc

@@ -32,7 +32,7 @@
 /*!
     \previouspage creator-visual-editor.html
     \page quick-projects.html
-    \nextpage quick-converting-ui-projects.html
+    \nextpage creator-qtquickdesigner-plugin.html
 
     \title Creating Qt Quick Projects
 

doc/qtcreator/src/qtquick/creator-only/qtquick-designer-plugin.qdoc

@@ -0,0 +1,55 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the Qt Creator documentation.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Free Documentation License Usage
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file. Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
+**
+****************************************************************************/
+
+/*!
+    \page creator-qtquickdesigner-plugin.html
+    \previouspage quick-projects.html
+    \nextpage quick-converting-ui-projects.html
+
+    \title Using \QMLD
+
+    We recommend that you use a separate visual editor,
+    \l{Qt Design Studio Manual}{\QDS} to open and edit
+    \l{UI Files}{UI files} (.ui.qml).
+
+    However, you can enable the \QMLD plugin in \QC for editing
+    UI files. The functionality is restricted and not all \QDS
+    features are supported.
+
+    To use \QMLD, switch to the \uicontrol Design mode when a ui.qml or or .qml
+    file is open.
+
+    For more information about using \QMLD, see \l{Qt Design Studio Manual}.
+
+    \section1 Enabling the \QMLD Plugin
+
+    To enable the \QMLD plugin:
+
+    \list 1
+        \li Select \uicontrol Help > \uicontrol {About Plugins} >
+            \uicontrol {Qt Quick} > \uicontrol {QmlDesigner}.
+        \li Select \uicontrol {Restart Now} to restart \QC and load the plugin.
+    \endlist
+*/