Docs: Add Oracle doc link to getting started page

Since there is a recommended version of JDK, it would be helpful to provide a link to the Oracle documentation. Since there are many versions of Java, those that are new or infrequent users of Java would find the link helpful. Thanks!

Closes #11792

docs/reference/getting-started.asciidoc

@@ -101,7 +101,7 @@ With that out of the way, let's get started with the fun part...
 
 == Installation
 
-Elasticsearch requires at least Java 7. Specifically as of this writing, it is recommended that you use the Oracle JDK version {jdk}. Java installation varies from platform to platform so we won't go into those details here. Suffice to say, before you install Elasticsearch, please check your Java version first by running (and then install/upgrade accordingly if needed):
+Elasticsearch requires at least Java 7. Specifically as of this writing, it is recommended that you use the Oracle JDK version {jdk}. Java installation varies from platform to platform so we won't go into those details here. Oracle's recommended installation documentation can be found on http://docs.oracle.com/javase/8/docs/technotes/guides/install/install_overview.html[Oracle's website]. Suffice to say, before you install Elasticsearch, please check your Java version first by running (and then install/upgrade accordingly if needed):
 
 [source,sh]
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