fixing documentation to be responsive

since requirejs documentation is pretty simple and clean, I have added nothing much needed to make it responsive..
as you can see, just made the side nav goes off canvas on small screens.

and to avoid any copyright issues, I used pure css to display the navigation buttons.

css is done in the same old way and pushed into the [requirejs-branding](https://github.com/jrburke/requirejs-branding) repo.

![requirejs-docs-off-canvas](https://f.cloud.github.com/assets/559002/1699171/7a688cca-5f86-11e3-9869-f8c6f9d81f5a.jpg)

fixes #840