Doc: Update documentation and snippets

The generated documentation include code from Qt.
Some of this code has been translated to Python, and it is currently placed in:

sources/pyside2/doc/codesnippets/doc/src/snippets/

Still, there are a few things to be addressed:

• Verify if all the Python code portions works
• Translate all the C++ references to Python
• Exclude the elements that we are not using, e.g.: QList<String> exists only as a Python string list, but we currently have documentation about it.

Additionally, some functions needs extra explanation, like QObject.findChild/find.Children and return types that are described as a python type that is not clear, e.g.: PyTuple.