Doc: Add more examples to Application Examples category

Add following examples to Application Examples category:

* Robot Arm Example
* Qt Quick Demo - RESTful Color Palette API client
* Media Player Example
* Document Viewer

Pick-to: 6.5 6.6
Change-Id: Ie36bb9a09d7fe868db07e02edcb1778f214494e9
Reviewed-by: Kimmo Leppälä <kimmo.leppala@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

examples/demos/colorpaletteclient/doc/src/colorpaletteclient.qdoc

@@ -4,6 +4,7 @@
 /*!
 \ingroup qtquickdemos
 \example demos/colorpaletteclient
+\examplecategory {Application Examples}
 \examplecategory {Networking}
 \meta tags {quick, network, http}
 \title Qt Quick Demo - RESTful Color Palette API client

examples/demos/documentviewer/doc/src/documentviewer.qdoc

@@ -5,6 +5,7 @@
     \title Document Viewer
     \ingroup examples-widgets
     \example demos/documentviewer
+    \examplecategory {Application Examples}
     \meta {tag} {demo,widgets,mainwindow}
     \brief A Widgets application to display and print JSON, text, and PDF files.
 

examples/demos/mediaplayer/doc/src/mediaplayer.qdoc

@@ -6,6 +6,7 @@
     \title Media Player Example
     \ingroup multimedia_examples
     \ingroup video_examples_qml
+    \examplecategory {Application Examples}
     \meta {tag} {quick,player,multimedia}
     \brief Playing audio and video using Qt Quick.
 

examples/demos/robotarm/doc/src/robotarm.qdoc

@@ -8,6 +8,7 @@
     \brief Demonstrates how to add a C++ backend to a 3D project from Qt Design Studio.
     This example demonstrates adding a C++ backend to a 3D project created in Qt Design Studio. The example itself consists of
     an interactive industrial robot arm in a Qt Quick 3D scene. The 2D UI to control the robot arm is implement using Qt Quick Controls.
+    \examplecategory {Application Examples}
     \meta {tag} {demo,quick}
     \image robotarm-example.png