'git mv' rst resources to md

This is mainly intended to keep the git history as neat as possible.
No actual conversion yet, so now both the rst and the paradox docs
are broken, which will be fixed in the next commits.

akka-docs/src/main/paradox/java/code/jdocs/extension/ExtensionDocTest.java

@@ -0,0 +1,89 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package jdocs.extension;
+
+//#imports
+import akka.actor.*;
+import java.util.concurrent.atomic.AtomicLong;
+
+//#imports
+
+import jdocs.AbstractJavaTest;
+import org.junit.Test;
+
+public class ExtensionDocTest extends AbstractJavaTest {
+
+  static
+  //#extension
+  public class CountExtensionImpl implements Extension {
+    //Since this Extension is a shared instance
+    // per ActorSystem we need to be threadsafe
+    private final AtomicLong counter = new AtomicLong(0);
+
+    //This is the operation this Extension provides
+    public long increment() {
+      return counter.incrementAndGet();
+    }
+  }
+
+  //#extension
+
+  static
+  //#extensionid
+  public class CountExtension extends AbstractExtensionId<CountExtensionImpl>
+    implements ExtensionIdProvider {
+    //This will be the identifier of our CountExtension
+    public final static CountExtension CountExtensionProvider = new CountExtension();
+
+    private CountExtension() {}
+
+    //The lookup method is required by ExtensionIdProvider,
+    // so we return ourselves here, this allows us
+    // to configure our extension to be loaded when
+    // the ActorSystem starts up
+    public CountExtension lookup() {
+      return CountExtension.CountExtensionProvider; //The public static final
+    }
+
+    //This method will be called by Akka
+    // to instantiate our Extension
+    public CountExtensionImpl createExtension(ExtendedActorSystem system) {
+      return new CountExtensionImpl();
+    }
+  }
+
+  //#extensionid
+
+  static
+  //#extension-usage-actor
+  public class MyActor extends AbstractActor {
+    @Override
+    public Receive createReceive() {
+      return receiveBuilder()
+        .matchAny(msg -> {
+          // typically you would use static import of the
+          // CountExtension.CountExtensionProvider field
+          CountExtension.CountExtensionProvider.get(getContext().getSystem()).increment();
+        })
+        .build();
+    }
+  }
+
+  //#extension-usage-actor
+
+  @Test
+  public void demonstrateHowToCreateAndUseAnAkkaExtensionInJava() {
+    final ActorSystem system = null;
+    try {
+      //#extension-usage
+      // typically you would use static import of the
+      // CountExtension.CountExtensionProvider field
+      CountExtension.CountExtensionProvider.get(system).increment();
+      //#extension-usage
+    } catch (Exception e) {
+      //do nothing
+    }
+  }
+
+}