DOC: freeze old whatsnew notes?
A problem with using the ipython directory for the whatsnew/release parts of the docs (so that all code examples are run during the doc build), is that after a while when things change/get deprecated, these pages don't build correctly anymore, generating warnings/exceptions.

An example: http://pandas-docs.github.io/pandas-docs-travis/whatsnew.html#hdfstore, with the `unique` attribute that was removed recently in #6813

I don't think we should start updating the older whatsnew docs (they are a kind of historical overview), but we should do something as those doc build warnings are annoying. A proposal:
- after a release (or after some time), we run whatsnew/release once with the pandas version of that release, and replace all `ipython` directory blocks to literal `code-blocks` with output.
- we maybe add a warning at the top of each whatsnew that the code examples can be outdated

An alternative is to add `:okexcept:` to those ipython code-blocks; this would solve the warnings during the doc build, but not the 'faulty' output in the docs itself.
