[Doc] Explain use of RDF models

[kenwenzel]

• show that RDF is well-suited to describe the structural (semantic) aspects of objects in the industrial domain (machines, parts etc.)
• show relationships to ontologies (IOF), OPC UA, ESMF SAMM <-> AAS etc.
• add example for inserting data via Graph store protocol using turtle or JSON-LD
• show that SPARQL can be used to answer common questions regarding the structure of digital twins