Replace multimodal export with datasets generation

The multimodal export example is still based on the old legacy format and it is not up-to-date with the latest datasets work.

We should:

1. Deprecate the function `docling.utils.export.generate_multimodal_pages`
2. Remove the example https://ds4sd.github.io/docling/examples/export_multimodal/
3. Add a new example which generates a datasets (see eval library)