Add requirements about datasets (like durable datastores)

[coret]

The current dataset requirements are about dataset descriptions. Not much is said about the contents of datasets (which one might gather from the title).

We might consider added requirements specificly for datasets, like:

• the preference of schema.org
• linked data
• the durability of data (read: explicitly say data from temporary/experimental datastores should be avoided).

[ddeboer]

We should also keep in mind https://netwerk-digitaal-erfgoed.github.io/cm-implementation-guidelines/#publishing-collection-information.

Should we add data requirements to the current Requirements for Datasets or create a separate document? Previously, we removed requirements relating to the dataset contents (#20).