Updates to Device Mode article

Re-organized device mode article to highlight key features and updated screenshots. 

A few things to note: 
* gifs might need to be resized to fit nicely into the article. (?)
* I also have high-res versions of the screenshots, but didn't incorporate them into this revision. (Wasn't sure if we had the necessary styling yet.) I can add them in next iteration. 

Let me know if I left anything out that you think I should have included. Thanks.

(@paulirish @Garbee @jaredwilli )