Remove archive pages from upgrade guide

while updating http://drupal.org/handbook/modules/update, i went to http://drupal.org/upgrade to see if i ought to add a link, and it wasn't in great shape, so i updated it as well (it could use a review).

more importantly though, the whole http://drupal.org/upgrade section is a bit of a jumble, good amount of redundant info that could be compressed into a more straightforward guide.