Describe SPARQL endpoint

[ddeboer]

Add example SPARQL and other queries (such as an HTTP URL to an OAI-PMH endpoint) to help users discover the data in the datasets.

For datasets that share a SPARQL endpoint, the information should include the way to select the dataset from that generic endpoint. We may use SPARQL Service Description for this:

[] a sd:Service ;
    sd:endpoint <http://www.example/sparql/> ;
        sd:namedGraph [
            a sd:NamedGraph ;
            sd:name <http://www.example/named-graph> ;
        ]
    ] .

An alternative approach is to use SHACL SPARQL targets:

ex:USCitizenShape
	a sh:NodeShape ;
	sh:target [
		a sh:SPARQLTarget ;
		sh:prefixes ex: ;
		sh:select """
			SELECT ?this
			WHERE {
				?this a ex:Person .
				?this ex:bornIn ex:USA .
			}
			""" ;
	] ;
	...