link to GT repo

[bertsky]

We have

• ocr-d.de/gt-repo
• ocr-d.de/en/data

still pointing to broken KIT metarepo (no actual data files) instead of Github.

IIUC this should now be https://ocr-d.de/gt_structure_text/ or https://github.com/OCR-D/gt_structure_text/releases.