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.

Start by reviewing the devguide documentation structure and the related CPython issue 95649, then ask VanL for the intended guidance. Document the license-compatibility, provenance, credits, Docs/license.rst, and commit-message requirements so the procedure is complete and actionable.