related_identifers: add resource_type docs

[slint]

We can now specify a resource_type for each related_identifiers entry like so:

{
    "metadata": {
        ...,
        "related_identifiers": {
            "identifier": "10.1234/foobar",
            "scheme": "doi",
            "relation": "cites",
            "resource_type": {
                "type": "dataset",
            }
        }
    }
}

This should be reflected in the documentation.