Doc: Make 'Qt Quick Effects' and 'Qt Quick Particles' linkable targets

These modules have no separate landing pages within Qt Quick. Add
\keyword commands to create linkable targets to their respective
QML module pages.

Link to Qt Quick Effects from the Qt Quick landing page.

Fixes: QTBUG-113454
Change-Id: I9376d9c946cc7a4d4728584a96ddf701deaeb4b5
Reviewed-by: Andreas Eliasson <andreas.eliasson@qt.io>
(cherry picked from commit 649f5f650b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/effects/qquickmultieffect.cpp

@@ -11,6 +11,7 @@ QT_BEGIN_NAMESPACE
 Q_LOGGING_CATEGORY(lcQuickEffect, "qt.quick.effects")
 
 /*!
+    \keyword Qt Quick Effects
     \qmlmodule QtQuick.Effects
     \title Qt Quick Effects QML Types
     \ingroup qmlmodules

src/quick/doc/src/concepts/effects/particles.qdoc

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
 /*!
+    \keyword Qt Quick Particles
     \qmlmodule QtQuick.Particles
     \title Qt Quick Particles QML Types
     \ingroup qmlmodules

src/quick/doc/src/qtquick.qdoc

@@ -123,6 +123,8 @@ To find out more about using the QML language, see the \l{Qt QML} module documen
     \li \l {Qt Quick QML Types}
            - a list of QML types provided by the \c{QtQuick} import
         \list
+            \li \l {Qt Quick Effects QML Types} {Effects}
+                   - provides graphical effects to apply to Qt Quick items
             \li \l {Qt Quick Local Storage QML Types} {Local Storage}
                    - a submodule containing a JavaScript interface for an SQLite
                    database