[TYPO3-doc] Review of doc_core_skinning

[Fabien Udriot]

Hi all,

Documentation about the CSS Guildlines / TYPO3 Skinning has been updated to reflect the latest 
changes in the Core. Basically, chapter "Sprite generation" and "Icon API" have been revamped 
besides an overall refresh.

There are two files in attachment:

- manual.sxw : the official documentation to be reviewed
- icon-reference.pdf: auto-generated file bringing together all icons from the Core into one 
document. This document will be available next to doc_core_skinning.

The document has been reviewed by Steffen Ritter.

I will be off for 4 days but don't hesitate to leave comments so that I can update it when I will be 
on line again.

Cheers,

Fabien