DOC: add 4th level to toc for various doc pages
[here](https://github.com/pydata/pandas/pull/10992) added 4th level menu items where appropriate in io.rst.

might pay to do this somewhat in some other sections just to break the sub-sections up a bit
- http://pandas-docs.github.io/pandas-docs-travis/dsintro.html

maybe other places as well
