Update Python bindings from XML to PYI

The [Create a Python Binding for C++](https://freecad.github.io/DevelopersHandbook/technical/CreatePythonBindingForCpp#class-description-xml-file) page describes the old approach with XML files, but currently PYI files are used.

Open the “Create a Python Binding for C++” page at the linked section and compare its class-description instructions with the issue’s note about PYI files. Update the page so it describes the current PYI-based approach instead of XML, then verify that the outdated XML guidance is gone.

Written by the indexing model from the issue text.