This rolls all "create missing" handling into BuildConfig, and moves the
build-dir option from the "book" table to the "build" table. Some
documentation cleanup surrounding the build table is also updated.
`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.