Add category for ground truth

[Konstantin Baierer]

• existing datasets, c.f. cneud/ocr-gt#1
• tools to create (archiscribe, glyph miner, ocr-gt-edit...)
• tools to evaluate

[Clemens Neudecker]

I moved the data from ocr-gt here to initiate a new datasets category with ground truth resources (and eventually further subcategories).

Tools to create/evaluate ground truth would imho better fit in a subcategory under the software category (as they partially already do).