Add mobile and tablet styles

[sch]

I found myself wanting to read the docs on my phone, but I was having a hard time! This PR makes minimal changes to the existing styles and as few changes as possible to the existing markup to add responsive views into the guides.

Screenshots

Mobile:

• masthead
• guide navigation
• guide text

Tablet:

• navigation
• guide text

Oddities

Some other things that should perhaps be addressed as a part of this feature:

• Should the guide navigation be collapsed in a menu on mobile? Heroku's docs do a nice job of this. Depends on your appetite for scrolling.
• Should the top navigation have some kind of menu? Stripe's thing is 👌. The search is probably the main offender here in my implementation.
• Anything you have in mind around the homepage?

[jwoertink]

I like this, and think it's pretty necessary. @sch, do you still have time to work on this? If so, can you rebase from master to pull in all the latest stuff?

[sch]

👍Just gave it a rebase. There are a few odd ducks from the new changes:

• inline code styles wrap badly
• the header is still cramped

I'll take a shot at improving those this weekend.

[jwoertink]

The code blocks probably shouldn't wrap. Maybe just give them an overflow? Thanks for hopping back on this!