slides: remove y-axis navigation

This makes the slides much easier to navigate: the slides exist on one axis to be viewed from left to right.

I'm not sure whether I've updated the right files, nor do I know whether this pull request should be against `master` or `gh-pages`. I don't see a makefile so I don't know whether there's a build step (and if there is, I don't know which commands to run). Let me know if I've missed a step so I can make amends. :)