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.

先檢視 devguide 文件的結構和相關的 CPython issue 95649，然後向 VanL 詢問預期的指引。記錄授權條款相容性、來源、致謝、Docs/license.rst 和提交訊息的要求，讓程序完整且可執行。