Docs: add code examples for drop and dropWhile Operators (#28613)

akka-docs/src/test/scala/docs/stream/operators/sourceorflow/Drop.scala

@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2019-2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sourceorflow
+
+import akka.NotUsed
+import akka.actor.ActorSystem
+import akka.stream.scaladsl.Source
+
+object Drop {
+
+  implicit val system: ActorSystem = ActorSystem()
+
+  def drop(): Unit = {
+    // #drop
+    val fiveInts: Source[Int, NotUsed] = Source(1 to 5)
+    val droppedThreeInts: Source[Int, NotUsed] = fiveInts.drop(3)
+
+    droppedThreeInts.runForeach(println)
+    // 4
+    // 5
+    // #drop
+  }
+
+  def dropWhile(): Unit = {
+    // #dropWhile
+    val droppedWhileNegative = Source(-3 to 3).dropWhile(_ < 0)
+
+    droppedWhileNegative.runForeach(println)
+    // 0
+    // 1
+    // 2
+    // 3
+    // #dropWhile
+  }
+
+}