Apache Forrest docs update

docs update
by
DC
:

Enhance the "HowTo Generate an ASF mirrors page" doc to better explain.
Refer to Forrest site-author as an example. This doc explains many things
which are useful beyond the "ASF mirrors" example. It shows how to
add a specialised HTML form to a forrest-based site, howto to include
and exclude non-linked extra documents to the processing using the
Cocoon CLI.