Doc: Add example categories for Qt Network examples

Task-number: QTBUG-116001
Pick-to: 6.5 6.6
Change-Id: I0c32df7752347f7c38996b3815d270dbd4c90707
Reviewed-by: Kai Köhne <kai.koehne@qt.io>

examples/network/doc/src/secureudpclient.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example secureudpclient
     \title DTLS client
+    \examplecategory {Networking}
     \ingroup examples-network
     \brief This example demonstrates how to implement client-side DTLS connections.
 

examples/network/doc/src/secureudpserver.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example secureudpserver
     \title DTLS server
+    \examplecategory {Networking}
     \ingroup examples-network
     \brief This examples demonstrates how to implement a simple DTLS server.
 

examples/network/doc/src/torrent.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example torrent
     \title Torrent Example
+    \examplecategory {Networking}
     \ingroup examples-network
     \brief Demonstrates complex TCP/IP operations.