Add a caution/warning box about actions/upload-artifact@v4 @ GHA publishing guide

[webknjaz]

So I want to have some warning displayed right before the readers first see actions/upload-artifact@v3 @ https://packaging.python.org/en/latest/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/#checking-out-the-project-and-building-distributions.

The text should tell people to avoid upgrading the action to v4 (and the same for the download one), if they build many platform-specific wheels, and not just a pure-python wheel+sdist.
The main conversation of how to live with the incompatible/breaking changes is happening @ actions/upload-artifact#472.

More context and some explanation is linked @ https://github.com/orgs/aio-libs/discussions/31. I didn't want to copy it over as is.