Doc: moved saxbookmarks example files

Moved to qtbase/examples/xml/saxbookmarks
Png and qdoc file.
Updated qtxml.qdocconf accordingly.

Change-Id: Iafe50a09aee22f518adf9ec2ebf289e766e24ff4
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>