Doc: Add example categories to Particles examples

Add the example categories of Particles examples.

Task-number: QTBUG-116265
Change-Id: Ie773c51c8a862c3cba74b22a68078d68eb14b42d
Reviewed-by: Mats Honkamaa <mats.honkamaa@qt.io>
(cherry picked from commit 0ebd825d10)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/quick/particles/affectors/doc/src/affectors.qdoc

@@ -4,6 +4,7 @@
 /*!
     \title Qt Quick Particles Examples - Affectors
     \example particles/affectors
+    \examplecategory {Graphics & Multimedia}
     \brief This is a collection of examples using Affectors in the QML particle system.
     \image qml-affectors-example.png
 

examples/quick/particles/emitters/doc/src/emitters.qdoc

@@ -3,6 +3,7 @@
 /*!
     \title Qt Quick Particles Examples - Emitters
     \example particles/emitters
+    \examplecategory {Graphics & Multimedia}
     \brief This is a collection of examples using Emitters in the QML particle system.
     \image qml-emitters-example.png
 

examples/quick/particles/imageparticle/doc/src/imageparticle.qdoc

@@ -4,6 +4,7 @@
 /*!
     \title Qt Quick Particles Examples - Image Particles
     \example particles/imageparticle
+    \examplecategory {Graphics & Multimedia}
     \brief This is a collection of examples using Affectors in the QML particle system.
     \image qml-imageparticle-example.png
 

examples/quick/particles/system/doc/src/system.qdoc

@@ -4,6 +4,7 @@
 /*!
     \title Qt Quick Particles Examples - System
     \example particles/system
+    \examplecategory {Graphics & Multimedia}
     \brief This is a collection of examples using Affectors in the QML particle system.
     \image qml-system-example.png