Simplify the polymer.dart homepage, move some stuff to the FAQ, tweak the FAQ.

https://polynav-dot-dart-lang.appspot.com/polymer/
https://polynav-dot-dart-lang.appspot.com/polymer/faq.html

This is a quick pass before 0.15 goes out. More to come in a future CL...

R=jakemac@google.com

Review URL: https://chromiumcodereview.appspot.com/601883003