Add information how to deal with incorporation external code (license, credits)

Describe the enhancement or feature you'd like
The devguide does not contain information what a triager and core dev has to do when adding files or non-trivial portions of code from an external project with non-core dev contributors. There are a couple of things we need to check or do:

• Is the license compatible with PSFL?
• Include proper provenance and license information in file header or comment before the code
• Add license and origin to Docs/license.rst
• Give credits to original authors
• Properly document the provenance and license in commit message

Describe alternatives you've considered
We should ask VanL for guidance, too.

Additional context
See https://github.com/python/cpython/issues/95649 for a case that is missing proper license and provenance information.

Commencez par examiner la structure de la documentation de devguide et l’issue 95649 correspondante de CPython, puis demandez à VanL quelles sont les directives prévues. Documentez les exigences relatives à la compatibilité des licences, à la provenance, aux crédits, à Docs/license.rst et aux messages de commit afin que la procédure soit complète et exploitable.