DOC modified sphinx's layout.hml

The previous sphinx layout relied on overriding a default template.
It meant we could not easily add links and other things in the header.
The new layout does not inherit from anything, hence makes it easier
to tune.

I've also tune a bit more the css: notice how the code is now in a round bordered rectangle, and the margins slightly bigger.

![matplotlib_new_design](https://f.cloud.github.com/assets/184798/728021/23e028d8-e1da-11e2-9ee6-b7b614286c35.png)