[racket] documentation example for slideshow/pict

From: Bill Richter (richter at math.northwestern.edu)
Date: Sat Aug 7 23:38:18 EDT 2010

Thanks, Shriram!  That sounds great, and I'm sure one of the Pict
experts could write better examples than mine.  But please look at the
slideshow & the keybinding dox in 3.3.8 Defining Custom Shortcuts.
See if you think if there's enough examples.  I know I floundered.

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