Add JSON-LD and UML overview

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I've received feedback that the spec quickly dives into the different Data/Contextual entities, but that there is no overview of them.

A suggestion was to add a UML diagram - I've used this one in some presentation, but not sure how suitable for website?

(I can make SVGs etc)

I've been thinking we should add something about the JSON-LD blocks and 'mini intro to JSON-LD':

So if we add such a page towards the beginning, right before https://www.researchobject.org/ro-crate/1.1/root-data-entity.html perhaps? That would also be a good place to mention how other schema.org types/properties can be used, but this spec highlights what we think are most useful.

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The second half (entities with arrows) has now been added to https://www.researchobject.org/ro-crate/1.2-DRAFT/introduction.html#ro-crate-root -- would the non-animated UML also fit there?