Clarify the ULAN predicate in the unmodeled relationship example

[sundog358]

Summary

The current unmodeled-relationship example describes Ferdinand Bol as a student of Rembrandt and recommends assigned_property when an appropriate predicate is known, but the generated example supplies only the display name.

This adds Getty's existing ulan1102_student_of predicate to that example. The direction matches the existing example: Ferdinand Bol is the subject carrying attributed_by, student of is the predicate, and Rembrandt is the assigned object. Using the full URI requires no additional compact-prefix declaration.

This is a narrow clarification of the existing pattern, not a proposal to add GVP properties to the Linked Art context.

Verification

• python -m mkdocs build --clean passes from a clean environment installed from requirements.txt.
• The generated JSON-LD and graph contain the full http://vocab.getty.edu/ontology#ulan1102_student_of URI.
• The same shape has a reproducible fixture and conformance test in Meta Museum's implementation note.