Export datasets in other formats (e.g. COCO format)

[nikos-livathinos]

Given that docling-eval is able to create ground truth and prediction datasets built around the DoclingDocument format we may also want to export the entire GT/prediction dataset in another format. This can be viewed as a utility functionality to assist other tasks.
For example some AI models require the training data in COCO format (https://cocodataset.org/#format-data).