select bounding box orientation on document save

A boundingbox already offers conversion methods

- https://docling-project.github.io/docling/reference/docling_document/#docling_core.types.doc.BoundingBox.to_bottom_left_origin
- https://docling-project.github.io/docling/reference/docling_document/#docling_core.types.doc.BoundingBox.to_top_left_origin

but these are not exposed when performing the OCR or when saving the document; i.e. `save_as_json` or other methods for saving the result do not offer a mechanism of changing the boundingbox orientation

## feature

allow for conversion of boundingbox orientation either during OCR or when saving the document