Documentation format on a GNUstep desktop

[Svetlana Tkachenko]

Hi all

In Windows there was CHM, in Linux there is rst and docbook (which can be 
output to a variety of other formats from such a source). 
What is the standard or common documentation format on Mac or a free equivalent?
Looking for a format that has searchable index, table of contents, and 
also for a viewer which can search across indices and full contents of multiple 
files.

Regards,
Svetlana