Add current sparql enpoint link to docs.

docs/interfaces/sparql.md

 # SPARQL endpoint
 
 UnHIDE exposes the full public part of the graph through a read only SPARQL endpoint:
-https://sparql.unhide.helmholtz-metadaten.de/.
+currently available under https://sparql.unhide.helmholtz-metadaten.de/sparql/ but in the future mainly under https://sparql.unhide.helmholtz-metadaten.de/.
 
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