Warn about artifact action behavior for wheel matrices

[Herrtian]

Adds a warning before the artifact upload/download workflow example for people adapting the guide to several platform-specific wheel jobs.

The sample now uses current artifact actions rather than the older v3 versions mentioned in the issue, so this keeps the caution framed around the still-relevant v4+ behavior: don't have multiple wheel jobs upload to the same artifact name; give each job its own artifact and adjust downloads accordingly.

Checks:

• git diff --check
• uv run --with pre-commit pre-commit run --files source/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows.rst
• uv run --with nox nox -s build

Fixes #1454

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📚 Documentation preview 📚: https://python-packaging-user-guide--2040.org.readthedocs.build/en/2040/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/#checking-out-the-project-and-building-distributions

[webknjaz]

I think this should be a tip, not a warning. That issue was filed years ago and nobody uses v3. So I don't see why we need to highlight it. Today, I'd rather have a shorter note along the lines of "when building multiple wheels, you'll end up needing using uniquely named artifacts per build job".

[Herrtian]

Thanks, agreed. I pushed c2b1179 to make this a short tip instead, without the old v3/v4 framing.

Checked locally with git diff --check, file-level pre-commit, and nox -s build.

[webknjaz]

Thanks!

[webknjaz]

Given https://www.dayswithoutgithubincident.com, it may take a few attempts to merge.

[webknjaz]

This one might actually be legit:

/home/runner/work/packaging.python.org/packaging.python.org/source/overview.rst:353: WARNING: unknown document: 'openstack:user/formats'

(though unrelated to the PR)

[webknjaz]

A rerun passed. So it was flaky.