docs: add example scheme programs

examples/factorial.scm

@@ -0,0 +1,13 @@
+; Tail-recursive factorial using a named let accumulator.
+(define (factorial n)
+  (let loop ((i n) (acc 1))
+    (if (= i 0)
+        acc
+        (loop (- i 1) (* acc i)))))
+
+(display "10! = ")
+(display (factorial 10))
+(newline)
+(display "20! = ")
+(display (factorial 20))
+(newline)