Introduce more flexibility for QA generation output persistence

Right now, the output file for QA generation [is specified in `GenerationOptions`](https://github.com/docling-project/docling-sdg/blob/main/docling_sdg/qa/base.py#L110), as used [in QA gen here](https://github.com/docling-project/docling-sdg/blob/main/docling_sdg/qa/generate.py#L234).

When using this to write a demo notebook in #11 and wanting to output multiple datasets from multiple documents in a 1:1 fashion, encapsulating the output method into `GenerateOptions` requires more complexity than allowing `generate_from_chunks` and friends to take a parameter specifying where to output, which is some usability friction.

A more general question is visiting which options should be in the `*Options` classes passed to constructors and which should be method parameters. Things like `api_key` definitely make sense where they are, but maybe others - `temperature` is another example - should probably have the existing defaults but also be overwritable in function calls.