nitdoc : New Overview page

nitdoc.nit :
This new overview page now display module list the same way that classes are displayed on a module page
 - The description is more complete
 - The graph is now on the top of the page, for a more instinctive view of the API
 - Added a sidebar with modules anchor for an easier navigation in the page

Nitdoc.UI.js :
Added two check to avoid javascript crashing on an overview page

Signed-off-by: itch76 <maxime.leroy76@gmail.com>