Adds documentation settings.

`uniform.docSettings` and `uniform.ghsettings` configure the `sbt-site` and `sbt-unidoc` projects to
create documentation for publishing under `target/site`. It will take the content from `src/site`
and also use `unidoc` to generate unified api documentation for the whole project.
is it possible to add mappings to hadoop? E.g., for Hdfs?