Adding a class for FAQs and other groups of links.

Adding a class to be used for FAQs and other lists of links for further reading. I call it ".moreinfo"

Satisfying a small part of Bug #994029. I will communicate the class name and usage to Chris. 

There is a testing page (https://developer.allizom.org/en-US/docs/User:stephaniehobson:moreinfo) that you can copy to your local dev if you like.
I picked 30px to match the left padding on our code samples, that padding is coming from prism which isn't managed in stylus. (I tried putting our normal gutter on it but it wasn't big enough for the flourish.)

The 15px was chosen separately because it looked right (and not because it's half for 30).