Add an option to SampleOptions to generate all samples

As noted in https://github.com/docling-project/docling-sdg/pull/38#discussion_r2242579276:

The demo notebook for conceptual generation uses a very big number to indicate that it wants ALL of the chunks in the documents. That works but it is clunky. It would be a bit cleaner to update `SampleOptions` have a way to *not* specify a limit, e.g., by specifying `None` for `max_passages` or something like that.