Broken links in helloworld_java.md

[mbrukman]

The latest version of documentation/codelabs/helloworlds/helloworld_java.md contains broken links at the very end of the document:

You've just completed the Soy Hello World using Java. Where should you go next?

• To use the same templates from this chapter in JavaScript, try the
  Hello World Using JavaScript examples.
• To read more about Soy concepts, take a look at the
  Concepts chapter.

Both of these links are broken:

• helloworld_js.md does not exist in the same directory (but would be very useful!)
• /third_party/java_src/soy/g3doc/concepts/index.md does not exist in this repo — should this point to
  /documentation/concepts/index.md instead?