Add explicit IDs to book's headers, make headers sentence case.

Also make sure all www.dartlang.org links outside the book start with /.

And add an image I happened to notice was missing.

Staged at:
https://bookids-dot-dart-lang.appspot.com/docs/dart-up-and-running/

R=shaza@google.com

Review URL: https://chromiumcodereview.appspot.com/706833002