use literalinclude directive to pull source directly from example package

generators/example/src/main/scala/ConfigMixins.scala

@@ -70,10 +70,12 @@ class WithDTMTop extends Config((site, here, up) => {
 /**
  * Class to specify a top level BOOM and/or Rocket system with PWM
  */
+// DOC include start: WithPWMTop
 class WithPWMTop extends Config((site, here, up) => {
   case BuildTop => (clock: Clock, reset: Bool, p: Parameters) =>
     Module(LazyModule(new TopWithPWMTL()(p)).module)
 })
+// DOC include end: WithPWMTop
 
 /**
  * Class to specify a top level BOOM and/or Rocket system with a PWM AXI4