Contrib modules documentation online

I've been working on converting the current README files for all contrib 
modules into sgml and add it to the documentation. There are still some fixes 
to do but i'd like to have some feedback. Indeed, it wasn't agreed to have 
all if any of the modules together with the core documentation.

You can see the docs on [1] in chapter VIII. If you think these could be a 
good addition, please fill free to comment on how you think sections should 
be organized to be consistent and easy to read.

[1] http://www.nan-tic.com/ftp/pgdoc