fix(document): save document images page by page

[ClemDoum]

Bug description

fixes: #179

When saving a markdown page by page (potentially to keep track of page size in characters for instance) images of the whole documents were save. This had 2 majors drawbacks:

• saving too many images when we want to dump a single page
• saving all images, n times for a n pages doc when saving it page by page

Fix

Changed

• updated DoclingDocument._with_pictures_refs to take the page_no and forward it to iter_items()

[mergify]

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