Documentation improvements

* Re enabling Java tests in akka-docs (they were not run before)
* Fixed bug #19764
* #19735 Rewrote every sample using the deprecated PushPullStage and friends
  using GraphStage
* Pruned old unused graph images
* Added missing graffle file for new graph images

akka-docs/rst/java/code/docs/stream/FlowGraphDocTest.java

@@ -13,6 +13,7 @@ import java.util.concurrent.TimeUnit;
 import akka.NotUsed;
 import akka.stream.ClosedShape;
 import akka.stream.SourceShape;
+import docs.AbstractJavaTest;
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -26,23 +27,24 @@ import akka.stream.*;
 import akka.stream.javadsl.*;
 import akka.testkit.JavaTestKit;
 
-public class FlowGraphDocTest {
+public class FlowGraphDocTest extends AbstractJavaTest {
 
   static ActorSystem system;
+  static Materializer mat;
 
   @BeforeClass
   public static void setup() {
     system = ActorSystem.create("FlowGraphDocTest");
+    mat = ActorMaterializer.create(system);
   }
 
   @AfterClass
   public static void tearDown() {
     JavaTestKit.shutdownActorSystem(system);
     system = null;
+    mat = null;
   }
 
-  final Materializer mat = ActorMaterializer.create(system);
-
   @Test
   public void demonstrateBuildSimpleGraph() throws Exception {
     //#simple-flow-graph