DEPR/DOC: rpy deprecation
so we should put up a deprecation notice on `rpy`imports that in the future the user should use  `rpy2` directly rather than the less-supported  `pandas.rpy` converters that we can take action say in 0.17.0

discussed extensivley in #7385 

xref #7385, #9187 #7309, #8255 
- [x] Update the docs with some example of how to port your pandas examples to the `rpy2` API -> done with #10075
- [ ] Go over the functionality in pandas and check if some things should be ported to `rpy2` (what still works better in pandas?)
  - [ ] Handling of NaN values (comment below: https://github.com/pydata/pandas/issues/9602#issuecomment-94716332)
  - [ ] index is not preserved in `ri2py` -> this should be handled in https://bitbucket.org/rpy2/rpy2/issues/285
- [ ] The closed PRs -> push them to `rpy2`?
