gh-123059: Document archive timestamp timezone handling

[JasonColapietro]

Closes #123059.

This documents the different timestamp behavior used when adding filesystem entries to ZIP and tar archives:

• ZipFile.write() converts the filesystem modification time to the current local time, which can make otherwise identical archives differ across time zones. The docs now show how to supply an explicit ZipInfo.date_time with writestr() when the timestamp must be controlled.
• TarFile.add() stores the modification time as epoch seconds without a local-time conversion, while noting that filesystem-provided timestamps may still be time-zone-dependent.

Checks:

• sphinx-lint on Doc/library/zipfile.rst and Doc/library/tarfile.rst
• targeted Sphinx HTML build with warnings treated as errors
• git diff --check

Developed with help from Codex. I reviewed and understand the change.

• Issue: zipfile and tarfile docs should cover local timezone impact on entry timestamps #123059

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Documentation build overview

📚 cpython-previews | 🛠️ Build #33915708 | 📁 Comparing 56fae89 against main (30ac23e)

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2 files changed

± library/tarfile.html
± library/zipfile.html