ENH: `styler.render.repr` option
offers option switching between "html" and "latex" output.

adds items to #41395.

also xref #39911 (technically after DataFrame.to_html is deprecated)

- [x] adds option
- [x] adds tests
- [x] updates options.rst docs
- [x] no public method docstrings  
- [x] whatsnew 

update to styler user guide as follow-on after all options are merged.

![Screen Shot 2021-08-29 at 10 04 24](https://user-images.githubusercontent.com/24256554/131243320-6a4d452c-9cdc-417d-ab20-54fffd8c3a76.png)

Have refactored this. This is the best, most generic pattern I think. When a `_repr_` function returns `None` iPython moves on to the next mimetype. So here we are manually specifying only one `_repr_mimetype_` to return a string. thanks I left the display issue open but xref