rename "annotation model" to "domain model" or "OLiA vocabulary"

[chiarcos]

Aside from linguistic annotations, OLiA now (2022) also provides models

• for grammatical features in machine-readable dictionaries (e.g., LexInfo), and
• for general linguistic terminology (BLL thesaurus)

The term "annotation model" is not really adequate for these applications.
The suggested change pertains to the documentation, only, but it sets it apart from all publications on OLiA so far. Its implementation should thus be aligned with milestone 1.0.