Doc: Add \examplecategory to qtgui

Task-number: QTBUG-116068
Pick-to: 6.5 6.6
Change-Id: Ia0bdc2122434adac51b37ba2207a2b0b993cf19b
Reviewed-by: Kai Köhne <kai.koehne@qt.io>

examples/gui/doc/src/rasterwindow.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example rasterwindow
     \title Raster Window Example
+    \examplecategory {Graphics & Multimedia}
 
     \brief This example shows how to create a minimal QWindow based
     application using QPainter for rendering.

examples/gui/doc/src/rhiwindow.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example rhiwindow
     \title RHI Window Example
+    \examplecategory {Graphics & Multimedia}
 
     \brief This example shows how to create a minimal QWindow-based
     application using QRhi.

examples/vulkan/doc/src/hellovulkancubes.qdoc

@@ -7,6 +7,7 @@
     \title Hello Vulkan Cubes Example
     \ingroup examples-vulkan
     \brief Shows the basics of using QVulkanWindow.
+    \examplecategory {Graphics & Multimedia}
 
     The \e{Hello Vulkan Cubes Example} shows more advanced usage of QVulkanWindow.
 

examples/vulkan/doc/src/hellovulkantriangle.qdoc

@@ -7,6 +7,7 @@
     \ingroup examples-vulkan
     \title Hello Vulkan Triangle Example
     \brief Shows the basics of rendering with QVulkanWindow and the Vulkan API.
+    \examplecategory {Graphics & Multimedia}
 
     The \e{Hello Vulkan Triangle Example} creates a full graphics pipeline,
     including a vertex and fragment shader, to render a triangle.

examples/vulkan/doc/src/hellovulkanwidget.qdoc

@@ -7,6 +7,7 @@
     \ingroup examples-vulkan
     \title Hello Vulkan Widget Example
     \brief Shows the usage of QVulkanWindow in QWidget applications.
+    \examplecategory {Graphics & Multimedia}
 
     The \e{Hello Vulkan Widget Example} is a variant of \l hellovulkantriangle
     that embeds the QVulkanWindow into a QWidget-based user interface using