Give main objects (Germplasm, Study, ....) a link to descriptive web page

[cpommier]

Use case : data exchange (graingene <->T3), data search (INRA, WUR, VID, iBet, ....) When fetching a json, for instance for germplasm, we need both

• access to related JSON (JSON-LD functionality)
• access to source database web page (card for the current germplasm)

[BrapiCoordinatorSelby]

I added a documentation URL field to high level objects, leaving the JSON-LD functionality for v2