Added introduction to Graphics AAM

[fredesch]

Copied most of the SVG AAM introduction to the Graphics AAM

[AmeliaBR]

Maybe don't need quite as much repeat, but not a big deal. However, at least two essential changes are required:

• The paragraph starting "This specification begins" needs to be removed and replaced with a description of the purpose of this specs.
• The list to other relevant specs needs to be updated to replace the reference to Graphics-AAM with a corresponding reference to SVG-AAM.

[fredesch]

@AmeliaBR wow, that is fast comments. I just noticed the reference to the Graphics AAM.