Add subtle link back to index

I've been working through these, but not necessarily in order. Sometimes I hope around, doing 2 or 3 in a row, and them I'm away from the index.

This adds a link back to [the Index][1] page under the `Next [...]` link:

![index link](http://cl.ly/image/3h413d3L2R0I/data)

I'm not attached to the wording, so suggestions welcome there. 

This commit is both the link and the resulting changes in `public` after running `tools/build`. The [README][2] doesn't mention if that should be the case. Since they are generated files, I would think they aren't to be tracked at all, but that may depend on how you're deploying the result... 

[1]: https://gobyexample.com
[2]: https://github.com/mmcgrana/gobyexample/blob/master/README.md