Add a Documentation directory inspired by the git one.

The html and manpages are generated from asciidoc source.  Short
description strings are extracted from the python modules.

Only the stg(7) frontpage and the stg-new(1) command page are already
written.  A template is provided for new command pages.

This also reworks a couple of help strings and command classification.

Signed-off-by: Yann Dirson <ydirson@altern.org>