manualpages: Add path and file extension to the configuration

manualpages: Add path and file extension to the configuration

This allows to correctly assign an URL to a file without relying
on unique base names.
A custom read function is required because General::readhash()
doesn't allow paths as hash keys. Modifying the existing functions
could affect other CGIs and was therefore dismissed.

Fixes: #12806
Signed-off-by: Leo-Andres Hofmann <hofmann@leo-andres.de>