[development] Theming documentation for developers

[Earl Miles]

I finally got around to writing proper documentation for the new theming 
system from the perspective of a module developer. This doc has never 
really existed in the past and even with the simpler theming system in 
Drupal 5 and previous, it probably should have, since it helps to 
encourage best practices.

Please see: http://drupal.org/node/165706