Add docs file layout

[vil1]

As per this issue in the plugin, we need to provide a minimal layout for documentation files, so that projects created with this template start with a working documentation microsite

[softinio]

What do you think about using one of these for it:

1. https://github.com/tpolecat/tut
2. https://47deg.github.io/sbt-microsites/

What do you think ? Or what do you have in mind?

[vil1]

Yes, exactly!