Doc: Use the \examplecategory macro to tag examples

The '\meta category' command was used for tagging examples with a
specific category, used in Qt Creators Welcome mode.

As we want to also generate lists of examples belonging to a category
elsewhere in the documentation, replace the command with a macro that
expands to the original \meta command and also adds the example to
a group using the \ingroup command. This way, the category names can
be used as arguments to the \generatelist or \annotatedlist commands.

Pick-to: 6.5
Task-number: QTBUG-112731
Change-Id: I38b38c524b35ad4c0049f2978f5fc65d03c6a1cd
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

examples/grpc/doc/src/chat.qdoc

@@ -4,7 +4,7 @@
 /*!
     \example chat
     \ingroup qtgrpc-examples
-    \meta category {Networking}
+    \examplecategory {Networking}
     \meta tag {network,protobuf,grpc,serialization,tcp}
     \title Chat
 

examples/grpc/doc/src/magic8ball.qdoc

@@ -4,7 +4,7 @@
 /*!
     \example magic8ball
     \ingroup qtgrpc-examples
-    \meta category {Networking}
+    \examplecategory {Networking}
     \meta tag {network,protobuf,grpc}
     \title Magic 8 Ball
 

examples/protobuf/sensors/doc/sensors.qdoc

@@ -5,7 +5,7 @@
     \example sensors
     \ingroup qtprotobuf-examples
     \meta tag {network,protobuf,serialization,udp}
-    \meta category {Networking}
+    \examplecategory {Networking}
     \title Sensors
 
     \brief The Sensors example shows how two applications can communicate by