3.0 Start new toc

The first of many changes for #1540.

In this set of changes I tried to do the following:

* Trim down the fat in the intro/overview sections.
* Try to give a good quick overview of the framework. I was inspired by [django's great overview](https://docs.djangoproject.com/en/1.7/intro/overview/).

The request overview is still hideous and is something I'll fix separately. Next I'll start restructuring the installation guide and tutorial and making it include some of the material that was removed from here.
I would remove the "a" at the start of this line.