documentation: update after major changes

 * Remove the huge section describing the design of the removed
   "project" feature

 * Update all references to the output directory hierarchy to match
   the new one

 * Add new sections on how to rebuild single packages and how to
   create board support

 * Other updates, clarifications, reorganizations

 Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>