[Translation] Start Vietnamese (vi) translation for Python Package Guide (CONTRIBUTING.po & index.po) #841

With the rapid growth of the Python and open-science community in Vietnam, many local researchers, students, and developers still face language barriers when adopting modern Python packaging standards. I would love to initiate and maintain the Vietnamese (vi) translation of the pyOpenSci Python Package Guide to make these best practices more accessible to the Vietnamese scientific and developer community.

Following the instructions in TRANSLATING.md, here is my proposed plan for the initial Pull Request:

1. Add "vi" to the languages list in conf.py.
2. Generate the initial Vietnamese catalog files under locales/vi/LC_MESSAGES/ using nox -s update-language -- vi.
3. Fully translate locales/vi/LC_MESSAGES/CONTRIBUTING.po (38 strings) or locales/vi/LC_MESSAGES/index.po (93 strings) in the first PR, keeping the scope small and easy to review.
4. Verify that nox -s build-language -- vi and nox -s build-all-languages-test build cleanly with zero warnings.

Please let me know if this plan sounds good to you. Thank you!