usage.rst is hard to follow for first-time users

When I first tried to use xhtml2pdf inside a Django app, I found the usage.rst to be incomplete and thus difficult to understand. I had to visit many discussion forums and read the questions and answers before I got a better understanding of this product.

To prevent other users from having to do the same digging around, I decided to offer @chrisglass to rewrite usage.rst from from the perspective of a first-time user.

The flow will be:

** Intro **
Basic usage (immediately show code)
PDF vs HTML (conceptual differences)
Defining page layouts (@page, @frame, Static frames vs Content Frames)
Flowing HTML content through Content Frames

** Advanced concepts **
Keeping text and tables together
Named page templates
Switching between templates
Using xhtml2pdf in Django

** Reference **
More details

I welcome people's input.

Thanks!
@lingthio