SequenceAnnotation example for a Gene Sequence

[AlasdairGray]

Add an example to the Gene profile showing how a Gene Sequence Annotation would be modelled.

Originally posted by @AlasdairGray in https://github.com/BioSchemas/specifications/issues/468#issuecomment-872355384

Outstanding issues:

• [ ] Example for Gene connecting to a sequence annotation (perhaps this can come from Ensembl @ens-ap5 ?)
• [x] Examples for the following properties on SequenceAnnotation
  • [x] sequenceOrientation
  • [x] sequenceValue