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.

Inizia esaminando la struttura della documentazione di devguide e la issue 95649 correlata di CPython, quindi chiedi a VanL quali siano le indicazioni previste. Documenta i requisiti relativi alla compatibilità delle licenze, alla provenienza, ai crediti, a Docs/license.rst e ai messaggi di commit, in modo che la procedura sia completa e attuabile.