ci: add per-PR XML structure check against doc-en

[lacatoire]

Adds a GitHub Actions workflow that runs on every pull request and checks that the block structure of modified XML files stays aligned with the corresponding doc-en file.

How it works

• Triggered on pull_request events targeting master
• Collects the list of .xml files changed in the PR
• For each file, reads the EN-Revision hash it declares and fetches doc-en at that exact revision
• Compares the element skeleton of both files (tag names and structural attributes, not prose)
• Emits ::error annotations in the PR if a divergence is found
• A file that lags behind sync is compared against the EN version it actually translated, so it does not produce false positives

Prior art

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