Document what Qt provides for serialization and what should be used in different use cases

With the introduction of an overhauled Overview documentation Qt is going to get close to 4 different API for serialization.

• https://doc.qt.io/qt-6/json.html
• https://doc.qt.io/qt-6/qdatastream.html
• https://doc.qt.io/qt-5/qtxml-module.html
• in a certain form https://doc.qt.io/qt-6/qtremoteobjects-index.html
• and soon Qt Protocol Buffers
• CBOR
• QSettings
  *

As the implementation is groomed and collected in one folder: 

https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/serialization

the documentation side is not that advanced... 

Qt Documentation has a page about serialization (https://doc.qt.io/qt-6/datastreamformat.html) but for some reasons, it does only speaks of Qt data types, whereas there are means for serialization with non-Qt data types too.

There were attempts to improve and generalize it:

https://wiki.qt.io/Rethinking_serialization_for_Qt6

And give an overview:

https://www.qt.io/blog/2018/05/31/serialization-in-and-with-qt

With Qt Protocol Buffers, serialization is different, since the use needs to define via proto files and cannot use Qt Protocol Buffers for already existing types.

As a PM I would like to make sure that Qt users will get an overview of that Qt provides for serialization and some guidance what should or should not be used in which cases

Additionally, it would be good to make sure that there are examples for all API sets. Some API, for example, QCbor does not have an example app. On the other side QJson provides an example app