Add a GitHub action to detect stale .po files and update mantainers

[flpm]

This PR sets up a simple way for translation maintainers to monitor when .po files (also called translation catalogs) may be stale and need to be updated.

Wait... why don't you just update the .po files in the action instead of detecting?

1. Updating .po files while contributors are working on them will cause merge conflicts when they submit a PR. While those conflicts are easy to fix (for a maintainer) they can be intimidating and scare away new contributors.

2. Also, it's easier to handle the merge of changes from the source into the .po file outside git because sphinx-intl uses msgmerge, which is po-format aware while the git merge driver is not.

What was done

• Added a script to detect changes to files in the English source of the guide
• Added a GitHub action that runs this script and when changes are detected posts a comment to issue Translation Maintenance Log #780
• Added tests to the script logic and a nox session to execute them

How to test

nox -s test-translation-scripts

Follow up work

The final piece (still to do) is to have updates to this issue be posted in the translation channel in slack.

AI Disclaimer

Claude code helped me with part of this, but I take full responsibility for the change.