`HtmlConfig` was both guaranteed to exist within `BookConfig`
and `expect`ed in few places.
This simplifies the API a little by representing the fact that
`HtmlConfig` is currently mandatory for proper mdBook binary operation.
- removing need to explicitly use `Path::new` all over the place
- removed warnings from doctests (normally invisible unless `cargo test -- --nocapture`)
- no doctests are norun/ignore now
- updated docs both in book-example and in docs not to refer to nonexisting API's
