DOC: in ReST, a sublabel defines it's parent. latex complains of dupe
This fix allows us to bring back the pdf documentation, albeit with some warnings.
I've uploaded the pdf to GH releases, and eventually hope to get it on the website.

Also updated the [release checklist](https://github.com/pydata/pandas/wiki/Release-Checklist) to make pdf publishing a standard part of our release process.

closes #5433
related #3518
