[racket] Documentation pointer request(s)

It seems that the user community would be well served by more convenient
links to HtDP.

Both the htdp.org and docs.racket-lang.org sites would benefit, I think, from
links to http://www.ccs.neu.edu/home/matthias/HtDP2e/, given its explicit
intention of updating with Racket releases. Right now I envision frustration
as users find documentation of software that has gently drifted with time.

-- 
Mason Loring Bliss             mason at blisses.org            Ewige Blumenkraft!
(if awake 'sleep (aref #(sleep dream) (random 2))) -- Hamlet, Act III, Scene I