Add vietnamese translation of sponsor.rst

[lamphung1]

This follows the contributor guide state that pytest welcomes documentation translations. I have created a separate directory (pytest/doc/vn), with a translated page for sponsors.rst.

No current English documentation was modified, and the Sphinx build is not impacted because "pytest/doc/vn" is not included yet.

Summary

** This PR adds a Vietnamese translation of the "sponsor.rst" documentation file.
** The change is documentation only, and doesn't include code changes.
** No changelog needed (per contributor guide for documentation changes)

[Pierre-Sassoulas]

How is this going to be read ? We need to include a translation mechanism in the existing doc instead, probably using gettext and the sphinx/readthedoc integration). And we need to create this mechanism before actually translating anything imo.

[RonnyPfannschmidt]

we made bad experiences with the Japanese translation, that just sat and got out of date - unless someone with a clue prepares a translation setup that includes tracking/updates we are in no position to accept translations for the docs

[nicoddemus]

Indeed, doc translation is a complete separate can of worms, which I'm not sure we have the resources to tackle and maintain.

Let's close this for now.