ci: add workflow to track doc-en changes and open sync issues

[lacatoire]

Adds a daily GitHub Actions workflow that watches commits on php/doc-en and automatically opens issues for files that need updating in the Japanese translation.

How it works

• Runs every day at 06:00 UTC (and can be triggered manually)
• Fetches all doc-en commits from the last 7 days
• For each commit, checks whether an issue already exists (deduplication by SHA)
• If not, opens an issue listing the JA files to update
• Skips commits tagged [skip-revcheck]
• Creates the sync-en label on first run if it does not exist yet

Prior art

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