Guide: intro edits

This is mostly organizational and simplifying. However, I also removed prose which I took to be too basic given that we expect this to be read by programmers. Comments welcome.

cc @steveklabnik 
I'm not sure what @steveklabnik is :+1: here. The change or @tomjakubowski's comment?

I wanted the text to be rendered like it is now. I got the idea from other instances of this syntax in the Guide and Reference (I didn't know the syntax was for blockquote). If it's wrong here, then the other instances need to change as well. Possibly the CSS needs to change in order to select on some other property so we can achieve similar rendered output...