DOC: ENH: Add a table of contents to the PDF documentation
For python packages that I use a lot I like to have a PDF version of the documentation on my tablet for reference.  The PDF version of the Pandas doc is far less useful than most because it does not have a Table of Contents in the sidebar.  This makes it extremely difficult to quickly look up anything, especially as the first 450 pages are the change log.   

Compare the [Pandas PDF](https://pandas.pydata.org/pandas-docs/stable/pandas.pdf) with [Numpy](https://docs.scipy.org/doc/numpy-1.11.0/numpy-ref-1.11.0.pdf) or [Scipy](https://docs.scipy.org/doc/scipy-0.16.0/scipy-ref-0.16.0.pdf) PDFs to see what I mean.
