[DOC] add example of rolling with win_type gaussian
Admittedly this is not the first issue I address, but this one's been open for several months now and so I figured I'd take it

- [x] closes #26462
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks @MarcoGorelli 