don't use Doxygen @mainpage command

[this issue refers to the CVS HEAD version, which i cannot select in the issue tracker]

rpg.module uses the @mainpage Doxygen command. this is bad, as it puts the following description on the index page of *all* contributes modules and themes (see http://drupal.kollm.org/doxygen/_contrib/drupal-contrib-phpdoc/ ). A suggested way of documenting a whole group of related Drupal modules (like RPG) is the use of Doxygen grouping. i fixed this once in CVS, but apparently this got overwritten in a later commit. please restore this change. thanks!