DOC: in ReST, a sublabel defines it's parent. latex complains of dupe

This fix allows us to bring back the pdf documentation, albit with some warnings and errors.
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