Use orjson for faster JSON serialization / deserialization

Considerations

• consider any minor transformations needed (e.g. AFAICT orjson's API seems to only include dumps and loads)
• take into account these options

Places to update

https://github.com/search?q=repo%3Adocling-project%2Fdocling-core+%28%22def+save_as_json%22+OR+%22def+load_from_json%22%29&type=code