Generate docgen output along with api_docs as part of the build

pkg/docgen/bin/sdk-introduction.md

-Welcome to the Dart API reference documentation,
-covering the official Dart API libraries.
-Some of the most fundamental Dart libraries include:
-
-* [dart:core](#dart:core):
-  Core functionality such as strings, numbers, collections, errors,
-  dates, and URIs.
-* [dart:html](#dart:html):
-  DOM manipulation for web apps.
-* [dart:io](#dart:io):
-  I/O for command-line apps.
-
-Except for dart:core, you must import a library before you can use it.
-Here's an example of importing dart:html, dart:math, and a
-third popular library called
-[polymer.dart](http://www.dartlang.org/polymer-dart/):
-
-    import 'dart:html';
-    import 'dart:math';
-    import 'package:polymer/polymer.dart';
-
-Polymer.dart is an example of a library that isn't
-included in the Dart download,
-but is easy to get and update using the _pub package manager_.
-For information on finding, using, and publishing libraries (and more)
-with pub, see
-[pub.dartlang.org](http://pub.dartlang.org).
-
-The main site for learning and using Dart is
-[www.dartlang.org](http://www.dartlang.org).
-Check out these pages:
-
-  * [Dart homepage](http://www.dartlang.org)
-  * [Tutorials](http://www.dartlang.org/docs/tutorials/)
-  * [Programmer's Guide](http://www.dartlang.org/docs/)
-  * [Samples](http://www.dartlang.org/samples/)
-  * [A Tour of the Dart Libraries](http://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
-  
-This API reference is automatically generated from the source code in the
-[Dart project](https://code.google.com/p/dart/).
-If you'd like to contribute to this documentation, see
-[Contributing](https://code.google.com/p/dart/wiki/Contributing)
-and
-[Writing API Documentation](https://code.google.com/p/dart/wiki/WritingApiDocumentation).