Making Open.GL mobile-friendly

These commits only affect the layout of Open.GL when visiting from a mobile device (screen up to 1024px).
I made sure nothing is changed when visited from a desktop browser, even with the 2nd commit (0974e7c), which moves some DOM elements.

I pushed the `#nav` under the `#content` on mobile screens because it seems more intuitive to me to show the next step once you're done reading, instead of having to go back to the top of the page to find the link.
If you don't want it *(maybe you'd rather add a sentence at the end of each article with a link to the next, like you did on the first ones)*, ~~you should accept this pull-request instead.~~ I tried, but it updated this pull-request instead. If you don't know how to to accept only one commit *(I'm not even sure it is possible)*, I can create a new fork with just the first commit.

Thanks for your awesome work, by the way, Open.GL helped me a lot :)