DOC: warnings and remove HTML
Couple comments in https://github.com/pydata/pandas/issues/11610#issuecomment-158411503
- warning in the whatsnew
- warning in the Styler docstring
- removes generated HTML version of the notebook, that's built as part of the doc build.

Building the docs now to double check, I'll merge  when those are good. 

Looks good!
 @jorisvandenbossche IIRC we have `ipython` as a requirement of the doc-builds, should also add `ipython-notebook` (or `jupyter`)
 in principle, it's not `ipython notebook`, but just ipython for <4 (nbconvert it included, so that is OK) or `nbconvert` for >= 4
 But maybe safer to use `ipython nbconvert` instead of `jupyter nbcnvert` in make.py (as the first one works for all versions, the jupyter one only for the more recent)
 @TomAugspurger did you also check the api docs for the new styler if they render work correctly?
 Yep, the warning shows up there, things look good. I can upload the generated version somewhere if you want.
 Ahh, shoot found one mistake.  I forgot to change `Styler.set` to `Styler.use` in `api.rst`. Is it easiest for you to change that?
 This is line 1713 in the current version of master, not sure how to link to it since GH renders the rst. `Styler.set` -> `Styler.use`
 no, you can go ahead and change it (I still have to switch from windows to linux for the final build)
