Adding example for collection operator (#29716)

* Adding example for collection operator

* Modifying example

* Add parenthesises to side effect only method

Co-authored-by: Johan Andrén <johan@markatta.com>

akka-docs/src/main/paradox/stream/operators/Sink/collection.md

@@ -13,6 +13,13 @@
 
 @scala[Collect values emitted from the stream into an arbitrary collection `That`. The resulting collection is available through a `Future[That]` or when the stream completes. Note that the collection boundaries are those defined in the `CanBuildFrom` associated with the chosen collection. See [The Architecture of Scala 2.13's Collections](https://docs.scala-lang.org/overviews/core/architecture-of-scala-213-collections.html) for more info. The [`seq`](seq.html) operator is a shorthand for `Sink.collection[T, Vector[T]]`.]@java[Operator only available in the Scala API. The closest operator in the Java API is [`Sink.seq`](seq.html).]
 
+## Example
+
+This example reads the numbers from a source and stores them in the List collection.
+
+Scala
+:   @@snip [Collection.scala](/akka-docs/src/test/scala/docs/stream/operators/sink/Collection.scala) { #collection }
+
 ## Reactive Streams semantics
 
 @@@

akka-docs/src/test/scala/docs/stream/operators/sink/Collection.scala

@@ -0,0 +1,23 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sink
+
+import akka.actor.ActorSystem
+import akka.stream.scaladsl.{ Sink, Source }
+
+import scala.concurrent.{ ExecutionContextExecutor, Future }
+
+object Collection {
+  implicit val system: ActorSystem = ???
+  implicit val ec: ExecutionContextExecutor = system.dispatcher
+  def collectionExample(): Unit = {
+    //#collection
+    val source = Source(1 to 5)
+    val result: Future[List[Int]] = source.runWith(Sink.collection[Int, List[Int]])
+    result.foreach(println)
+    //List(1, 2, 3, 4, 5)
+    //#collection
+  }
+}