BUG: excelwriter engine_kwargs fix
- [x] closes #29095
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them - *I cannot find the linked page*
- [ ] whatsnew entry


The `engine_kwargs` argument was not passed properly and this was not tested. Also added some documentation.