move detailed setup documentation to interface modules

Only keep basic instructions for Lighttpd+mod_fastcgi and
Apache+mod_perl in PLP's main synopsis. Other configurations is given in
the pods of interface modules. These are linked at PLP#DESCRIPTION,
which now also contains an overview of supported servers.

Mod_perl configuration variables have also been moved into PLP::Apache.
In all, makes the main documentation much cleaner (uncluttered with
interface-specific details).