Add in-depth guide for ngAria

To help explain what the ngAria module can and can't do, I've updated the Developer Guide to include more in-depth information, code examples and accessibility resources. This is a companion piece to a blog post on the official AngularJS blog due out shortly.

Related to https://github.com/angular/angular.js/issues/9925
could we wrap these lines at 100cols so it's a bit easier to review?