Doc: Describe integration with Qt Marketplace

There is now a Marketplace tab in the Welcome mode.

Task-number: QTCREATORBUG-23452
Change-Id: I8043adb3104d542b63d5963261e92d7044222204
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

doc/qtcreator/src/howto/creator-ui.qdoc

@@ -51,6 +51,9 @@
 
         \li Open tutorials and example projects
 
+        \li Browse Qt extensions in the \l{https://marketplace.qt.io/}
+            {Qt Marketplace}
+
         \li Read news from the online community and Qt blogs
 
         \li Create or manage a Qt Account

doc/qtcreator/src/overview/creator-only/creator-configuring.qdoc

@@ -177,4 +177,8 @@
     To enable and disable plugins, select \uicontrol Help >
     \uicontrol {About Plugins}.
 
+    The \l{https://marketplace.qt.io/}{Qt Marketplace} contains \QC plugins
+    that you can download and install either for free or for a price set by
+    their publisher. You can browse the available plugins in the
+    \uicontrol Marketplace tab in the Welcome mode.
 */

doc/qtcreator/src/overview/creator-only/creator-tech-support.qdoc

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2019 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the Qt Creator documentation.
@@ -62,6 +62,11 @@
             \li \l{https://wiki.qt.io/Qt_Contribution_Guidelines}
                 {Qt Contribution Guidelines}
 
+       \row
+            \li Find extensions for Qt, such as \QC plugins, development tools,
+                and Qt modules
+            \li \l{https://marketplace.qt.io/}{Qt Marketplace}
+
         \row
             \li Find free Qt-based applications
             \li \l{https://github.com/topics/qt}{Qt Apps on GitHub}