Good default index page

Migrated issue

Reported by: ax

currently, api.module requires a @mainpage somewhere in the code to show *any* documentation. this has (at least) 2 drawbacks:

1) authors of contributed modules and themes wanting to use api.module to document *only their module/theme* have to put a @mainpage somewhere in their module/theme to see documentation. problem: running api.module on the whole contributions repository finds many @mainpage commands, and puts a wrong one on the repositorys main page (see http://drupal.kollm.org/doxygen/_contrib/drupal-contrib-phpdoc/ ). not so nice.
2) it is incompatible with Doxygen. in Doxygen, @mainpage is used to /customize/ the index page, not to /define/ it. if there is no @mainpage, Doxygen just generates a default (empty) one. in any case, the documentation is accessible.

please remove this requirement. thanks!