doc_layout : added DocSidebar, DocSidebox, DocSideboxGroup and DocListElement classes


Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
nitdoc : use doc_layout for sidebar html generation on an overview page

moreover, there is some commented work in progress in NitdocModule class,
this shouldn't be considered.

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>