[Enhancement] Integrate Mangrove Review Standard

[CMiksche]

Is your feature request related to a problem? Please describe.
Currently this project is based on ActivityPub, however ActivityPub wasn't designed to be a review standard.

For a better way to share the Review data I would suggest that a decentralised review standard should be implemented additionally.

Describe the solution you'd like
The Mangrove Review Standard was designed as a decentralised review standard and supports books, urls, restaurants and more. See https://mangrove.reviews/standard for the Specification.

Mangrove is designed by a non-profit, the Open Reviews Association and is also basically decentralized - meaning every BookWyrm Instance could also be a Mangrove instance.

Additionally BookWyrm could benefit from other applications where users rate the books and the other way around other apps would benefit from BookWyrm reviews - so we would help creating a better decentralised ecosystem for reviews.

Additional context
To give a example, let's say I create a App which lets users see Reviews of books. With the Mangrove Review Standard I could easily integrate multiple sources of the Reviews, if I just rely on ActivityPub I guess that every Instance has a slightly different definition of how a book looks like as a ActivityPub Object?