Add metadata to serialization examples

There's no separate category for {de,}serialization, so I've put them
in Input/Output; and at least some of them are relevant to
transmission over the network, so include that as a tag, so that
someone writing a network application that wants to use what they
illustrate gets a chance of being lead to it.

Pick-to: 6.5
Task-number: QTBUG-111228
Change-Id: Ie9f6dbaf85aed852e9f64eba05ec8c5fe0a7382d
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

examples/corelib/serialization/cbordump/doc/src/cbordump.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example serialization/cbordump
     \examplecategory {Input/Output}
+    \meta tag {network}
     \title Parsing and displaying CBOR data
 
     \brief A demonstration of how to parse files in CBOR format.

examples/corelib/serialization/convert/doc/src/convert.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example serialization/convert
     \examplecategory {Input/Output}
+    \meta tag {network}
     \title Convert Example
 
     \brief The Convert example demonstrates how to convert between different

examples/corelib/serialization/streambookmarks/doc/src/qxmlstreambookmarks.qdoc

@@ -3,6 +3,8 @@
 
 /*!
     \example serialization/streambookmarks
+    \examplecategory {Input/Output}
+    \meta tag {network}
     \title QXmlStream Bookmarks Example
     \examplecategory {Input/Output}
     \brief Demonstrates how to read and write to XBEL files.