DOC: more modern sphinx theme
We currently use a slightly adapted version of the 'nature' theme (you can also see it in this list of built-in themes: http://www.sphinx-doc.org/en/stable/theming.html#builtin-themes).

I think this can use an update, for some reasons:

- a mobile friendly theme (more and more useful, and currently this completely fails on mobile)
- more powerful theme (allowing eg better nested structure of the docs, which could enable a reorganisation of the docs. Now we have a single flat table of contents on the left)
- refreshed look (but this is subjective of course :-))

(could go together with a refresh of the main website as well, but not necessarily I think)

http://www.writethedocs.org/guide/tools/sphinx-themes/

