* Remove duplicate Review object under `rating` property
This was preventing validation, since `rating` is not a valid property
(`review` is, which is created from book.html already).
* Drop `bestRating` property in ratings, since it defaults to 5
See <https://schema.org/bestRating> ("If bestRating is omitted,
5 is assumed").
* Create Rating object (and its enclosing Review) in book/rating.html
* Use `position` property for Book objects in a series
`volumeNumber`, previously used, is only valid for objects of type
PublicationVolume (which series members are not).
* Give URL of book series when setting of `isPartOf`
* series.html: Add empty BookSeries object
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Co-authored-by: Adeodato Simó <dato@users.noreply.github.com>
