Pickling docling docling_parse.pdf_parsers.pdf_parser_v2

### Requested feature

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It would be nice to pickle the objects, then we can jsut save them and load them when preprocessing instead of preprocessing everytime.
conv_res = doc_converter.convert(abs_file_path)

Start with the docling_parse.pdf_parsers.pdf_parser_v2 code reached by doc_converter.convert(abs_file_path), and trace which objects are produced during preprocessing. Clarify the serialization and reload lifecycle before changing it; the work is done when the relevant objects can be pickled, saved, loaded, and reused instead of preprocessing every time.

Written by the indexing model from the issue text.