Rename manual.scrbl to rackjure.scrbl

This avoids doc name conflict if the package is installed at
installation scope rather than user scope.

See <http://lists.racket-lang.org/dev/archive/2014-July/014465.html>.

In makefile, use --tidy with raco setup to clean up the doc folder under
the old name. This is what raco pkg update will do, for users.