ci: add per-PR EN-Revision hash check

[lacatoire]

Adds a GitHub Actions workflow that runs on every pull request and verifies that the EN-Revision header in modified XML files points to the latest doc-en commit for that file.

How it works

• Triggered on pull_request events targeting master
• Collects the list of .xml files changed in the PR
• For each file that also exists in doc-en, reads the declared EN-Revision hash
• Compares it against git log -1 on the matching doc-en file
• Emits a ::error annotation and fails if the hash is missing or outdated

This ensures that translation updates always reference the exact upstream commit they are based on, making future sync work easier to track.

Prior art

This check is already in use in doc-fr, doc-ru, and doc-es. There is also an ongoing initiative in doc-base to provide reusable workflows that all doc-* repos can share.