docs: reduce noise on configuration file

IMHO, the generated sphinx config file is too noisy and thus hard to
manage. Let's clean it up.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
docs: show documentation for both the class and its __init__ method

Signed-off-by: Cleber Rosa <crosa@redhat.com>
docs: add initial man page

This page is intended as a basic offline aid for new users of avocado.

Signed-off-by: Cleber Rosa <crosa@redhat.com>