Book page: add Dart Cookbook, remove "What is Dart?", update issue tracker URL

Staged at:
https://booklist-dot-dart-lang.appspot.com/books/

BUGS= https://github.com/dart-lang/www.dartlang.org/issues/1102
      https://github.com/dart-lang/www.dartlang.org/issues/1113

R=sethladd@google.com

Review URL: https://chromiumcodereview.appspot.com/687733010