Adjust table of contents in the main page

[ellenkoenig]

http://opentechschool.github.io/python-data-intro/

I think it makes sense to do this on the fly, i.e. as we change content. It's also worth noting, for clarity's sake, that there are three main areas to pay attention to when adding, removing, or otherwise changing content:

1. The TOC menu on the left-hand side of every page, which can be modified in _config.yml.
2. The content overview on the main page, which can be modified in index.md.
3. The "Next Chapter" section on each page of the core content.