[WIP] Add DefinedTerm profile

[sneumann]

Internal reference

Fixes https://github.com/BioSchemas/specifications/issues/652

Description

• Adding validation, better description and examples for ontology and terminology service backed DefinedTerms

Motivation and context

See https://github.com/BioSchemas/specifications/issues/652

Have these been tested?

• e.g. New draft tested against JSON document...

What should reviewers focus on?

Since no new properties were added on top of the SchemaOrg type, only the description, marginality and cardinality need checking.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New content (non-breaking change which adds new content)
• [ ] Modified content (non-breaking change which modifies existing content)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Future TO-DOs

• [ ] Ping resources to update their DefinedTerms to this profile