List links to other language doc pages in alphabetical order (and add link to C++)

Please update drop down menu of [Java docs page(s)](https://googlecloudplatform.github.io/google-cloud-java/0.46.0/index.html)

### Expected Behavior

.NET, C++, Go, Java, Node.js , PHP, Python, Ruby

### Actual Behavior

.NET, Go, Java, Node.js, PHP, Python, Ruby

![image](https://user-images.githubusercontent.com/6241635/39551793-7475a7c0-4e34-11e8-8324-230e378af87b.png)
