Doc style and structure improvements

sphinx-better-theme 0.13 has the following relevant features:

* Better use of whitespace
* Better behavior on mobile devices
* Show actual titles of previous and next pages, instead of "next" and "previous" (also removes the "index" and "module index" links from the nav bar)
* Integration of some styles previously custom to mrjob

Changes made to mrjob as a result:

* Reorder the ToC to have a better linear order, so you could keep clicking the "next" link and have it make sense
* Remove now-redundant styles (Only changes the colors of the links now!)
* Link to index and modindex under the ToC since they are no longer in the nav bar (basically I have appropriately demoted their importance in the layer).

PyPI seems to be having some trouble today, so I'm going to wait to merge this until you can actually install sphinx-better-theme 0.13.