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 think that's one way of taking it. The way I was thinking when I first wrote this was that you don't need `gcc` installed to run a C program, you just need its dependencies. If you use Nokogiri with Ruby, you still need libxml installed. The difference is that you _also_ need Ruby installed.