Pospell check

[m-aciek]

Add automated spelling checks for Polish documentation using pospell and Hunspell.

The workflow runs on pull requests and manual dispatches against matching CPython documentation sources. It also runs after translation updates on scheduled/default-branch runs, where failures are advisory and do not fail update-and-build.

• Combine the packaged Polish dictionary, a checksum-verified SJP dictionary, and Polish terminology downloaded from the Transifex glossary.
• Convert dictionaries to UTF-8.
• Regenerate temporary catalogs with literal blocks excluded and obsolete entries removed. No generated PO changes are committed.
• Use GitHub's self-repository workflow syntax, with a narrowly scoped actionlint exception pending upstream support.

The PR workflow reaches pospell and reports findings; the standalone PR check currently fails on those findings.

Closes #75.