move demo documents into separate files

demo/docs/cpp.cpp

@@ -0,0 +1,13 @@
+// compound assignment operators
+
+#include <iostream>
+using namespace std;
+
+int main ()
+{
+    int a, b=3; /* foobar */
+    a = b;
+    a+=2; // equivalent to a=a+2
+    cout << a;
+    return 0;
+}