[Docs, CSS FrontEnd, Responsive] Improve drop-down menu on small width
The dropdown menu in Arraymancer docs has issues:
 ![image](https://user-images.githubusercontent.com/22738317/34068671-adb45a2e-e23f-11e7-8621-81cdf80c1a52.png)

 1. Not responsive: when the width is reduced, items like "Under The Hood" disappear
     ![image](https://user-images.githubusercontent.com/22738317/34068732-edd0ebe4-e240-11e7-874a-354ea8b884e9.png)

    
 2. The "Technical reference" should be the last item, but when the space on the right is not enough, it stacks the menu. It is then not possible to navigate from the `Core Tensor API` to its listed items without triggering Neuralnet primitives and its listed items
     ![image](https://user-images.githubusercontent.com/22738317/34068717-bf9ba2dc-e240-11e7-88db-9d6710f0cc6c.png)


The docs HTML is generated here:
https://github.com/mratsim/Arraymancer/blob/d7f28305998925adbdf38b3fee77ce40b573e063/nimdoc.cfg#L32-L126

And the CSS is here:
https://github.com/mratsim/Arraymancer/blob/d7f28305998925adbdf38b3fee77ce40b573e063/docs/nav.css#L30-L122
