Make CSS path relative.

We have hapi in a subdirectory (https://api.example.com/v1). With the default configuration lout sets the endpoint to `/docs` making the documentation available at https://api.example.com/v1/docs. The problem is that this same endpoint (`/docs`) is used for the CSS path, which results in a link:

    <link href="/docs/css/style.css" rel="stylesheet">

This link expands to https://api.example.com/docs/css/style.css which results in 404 (`/v1` is missing in the URL). So I propose to make the CSS path relative to the endpoint route. If endpoint is `/docs`, then set CSS path to `docs`. Then link will become:

    <link href="docs/css/style.css" rel="stylesheet">

This expands to https://api.example.com/v1/docs/css/style.css which is the correct URL.