Setup automated ScalaDoc publishing on every release

[bzz]

As part of the release, publish an auto-generated ScalaDoc website.

We used to have an in-house solution for that integrated as part of src-d/ci, but now that it's not maintained any more we step away from it.

We could simply use Github pages as hosting here and automation can be done with:

• https://github.com/sbt/sbt-site with a plugin
• https://github.com/sbt/sbt-ghpages