[doc] fix warnings/errors in doc build - fix links to images - fix a couple of typos - re-add IDownloadableOneLineView doc - rename documenting.rst back to .txt, it's intended as a doc of how to write rst, not part of the rst doc Related to #4832808

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0