Add schema.org description of datasets so they can be indexed by Google

[joaquinvanschoren]

Guidelines here: https://developers.google.com/search/docs/data-types/dataset

[HeidiSeibold]

Do the data packages use schema.org description? Then we could solve two problems at once.

See #482 (and https://github.com/datahq/datahub-qa/issues/33#event-1805583144)

[mitar]

Newer link: https://schema.org/Dataset