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.
We could roll this up into the previous selector with `&:last-child` like this:

```
dd {
    margin-bottom: 10px;

    &:last-child {
        margin-bottom: 0;
    }
}
```