[documentation] Theme developers guide overhaul

[Heine Deelstra]

Here's a sketchy outline for the proposed theme docs.

- Introduction
   - How Drupal generates output
   - Themes, engines & styles
   - Regions
- PHPTemplate
   - Introduction to PHPTemplate (explains the basics, default overrides  
etc)
   - page.tpl.php (explanation on the basic page)
     - available variables (list of available variables and explanation)
   - node.tpl.php
     - idem
   - block.tpl.php
     - idem
   - comment.tpl.php
     - idem
   - boxes.tpl.php
   - Making extra variables available
   - Overriding theme functions (including clear explanation of arguments &  
argument passing etc)
   - Examples
     - Howto: do something specific
     - Snippets (?)
   - Updating your theme
- Other engines
   - XTemplate
   - PHPTal
     - idem (phptemplate)
   - Smarty (etc)
     - idem
- List of Forms & functions to override for drupal core, including  
arguments / description
- Contributing a theme to Drupal.org
   - Adding your theme to Drupal.org
   - Screenshot guidelines
   - License