docs: add example scheme programs

examples/fib.scm

@@ -0,0 +1,15 @@
+; Naive recursive Fibonacci.
+(define (fib n)
+  (if (< n 2)
+      n
+      (+ (fib (- n 1)) (fib (- n 2)))))
+
+(display "fib 10 = ")
+(display (fib 10))
+(newline)
+(display "fib 20 = ")
+(display (fib 20))
+(newline)
+(display "fib 25 = ")
+(display (fib 25))
+(newline)