Document the "Document info" method

https://uploadcare.com/api-refs/rest-api/v0.7.0/#tag/Conversion

https://github.com/uploadcare/pyuploadcare/issues/258#issuecomment-1791593370

It is implemented, but not documented as a separate method.

See https://pyuploadcare.readthedocs.io/en/latest/core_api.html#document-conversion:
```converted_group: FileGroup = file.get_converted_document_group(DocumentFormat.jpg)```