move code to src/test

* so that it compiles and tests pass
* fix some additional snip references in getting started

akka-docs/src/test/scala/docs/CompileOnlySpec.scala

@@ -0,0 +1,13 @@
+/*
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+
+package docs
+
+trait CompileOnlySpec {
+  /**
+   * Given a block of code... does NOT execute it.
+   * Useful when writing code samples in tests, which should only be compiled.
+   */
+  def compileOnlySpec(body: => Unit) = ()
+}