DOC: add 4th level to toc for various doc pages

[jreback]

here 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

[jorisvandenbossche]

Now, in general, I would rather keep it to 3 levels, and split it in multiple files instead of keeping expanding the current files. See #10446

(but just for now making it better navigatable, having 4 levels is OK, for the long term I would go with #10446)

[jreback]

yes, it IS prob better to simply split io.rst up. Though it will still need to be 4 levels even if we do that :<

[jreback]

though was thinking in the Intro section, their are a lot of bullet points,, this is just some organization to it .... my 2c

[jorisvandenbossche]

Let's close this, we have issues about splitting that page, and the new docs theme will try to handle this case better as well.