Function converts a gtsummary object to a knitr_kable object.
This function is used in the background when the results are printed or knit.
A user can use this function if they wish to add customized formatting
available via knitr::kable.
Output from knitr::kable is less full featured compared to
summary tables produced with gt.
For example, kable summary tables do not include indentation, footnotes,
or spanning header rows.