Move docs to root folder

When we place `docs` in the root folder, we gain direct access to it even when we start a regular application server. It worked before as well, but a plain route to `/docs` is nicer than `/dist/docs`.

This means that every deployed version of the application will automatically ship together with the documentation (the necessary update in [arethusa-cli](http://github.com/latin-language-toolkit/arethusa-cli) is pending).

Fixed path to our `plato` report in the process.