Improve the CSS

[GaelVaroquaux]

• Check that it renders well on mobile phone
• Use PureCSS for the above, and because it looks cooler
• Add a "table of contents" that (models are http://nilearn.github.io/introduction.html#python-for-neuroimaging-a-quick-start http://bokeh.pydata.org/en/latest/, the design of bokeh being nicer):
  • Doesn't scroll down
  • Highlights the current section
  • Disappears on small screens (mobile friendly + tutorial friendly)

[NelleV]

I can work on this.

[raquelalegre]

I can help as well

[GaelVaroquaux]

The sidebar is now implemented in master. Mobile work OK. Closing.