Introduce a DoclingDocument Ground Truth DatasetBuilder

[nikos-livathinos]

• Introduce DoclingDocumentDatasetBuilder to build Ground Truth datasets from lossless serializations of DoclingDocument files (e.g. jsons).
• It is useful when DoclingDocument objects have been produced not as a result of a Docling conversion pipeline but by an one-to-one translation of other annotation formats (e.g. convert WordScape annotations to DoclingDocument format).
• It can extend the existing FileDatasetBuilder.