Broken links in helloworld_java.md

The latest version of [`documentation/codelabs/helloworlds/helloworld_java.md`][helloworld_java] 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](helloworld_js.md) examples.
> - To read more about Soy concepts, take a look at the
> [Concepts](/third_party/java_src/soy/g3doc/concepts/index.md) 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?

[helloworld_java]: https://github.com/google/closure-templates/blob/e42698a54f4319509e3f66d16585da3281c0f81b/documentation/codelabs/helloworlds/helloworld_java.md