Structured Data Extraction w/ Pre-defined Schema - Support for Excel/Spreadsheets

### Requested feature

Docling recently added support for structured data extraction using a predefined schema (see [here](https://www.linkedin.com/posts/peter-w-j-staar-7b261373_opensource-ai-documentai-activity-7368962587150958593-d26D?utm_source=share&utm_medium=member_desktop&rcm=ACoAABCJPgsBdh3vsqTOnNirJ6WNnM1_nIN9CQs)), but only currently supports pdfs & image files. Adding support for Excel/spreadsheet files would be extremely useful for myself and (I imagine) many others. I understand this feature is still in beta, but I am willing to try to implement Excel/spreadsheet file support myself and make a PR if the Docling team supports this.

### Alternatives

There are several other libraries that do this, most of them direct competitors with Docling (Llamaindex, Unstructured, etc). However, most of those competitors have only limited open-source/free options; they typically require using their paid API for the most effective solutions.

### Conclusion
I am a firm believer in open-source software, and I believe adding this feature to Docling would benefit the project tremendously and encourage many users to use Docling over other closed-source competitors. As I mentioned above, I am willing to try to implement this myself and make a PR if the Docling team supports this idea.

If anyone has suggestions on how to go about implementing this, or feedback, questions, etc., please let me know.

Thank you!