Cleans up links in Cookbook Intro

- Fixes link for "topic based guide" to point to the Ember.js Guides instead of a 404 Error page
- Fixes link for "API documentation for a class" to point to an actual class (`Ember.Application`) instead of the top-level API page
- Removes unused reference/footnote links.
Merge pull request #1724 from jarrodldavis/patch-1

Cleans up links in Cookbook Intro