DOC: Removing rpy2 dependencies, and converting examples using it to regular code blocks
- [X] closes #20648
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Still under discussion (https://github.com/pandas-dev/pandas/issues/20648#issuecomment-439343998), but in this PR I'm removing the rpy2 documentation page, as I think that documentation should live in rpy2 as it's not pandas documentation, and at the moment we have 50 additional dependencies for a small page in which examples were broken for some versions (and no user complained about it afaik). 