Adopt semantic versioning

[aclayton555]

The HTAN 1.0 data model adopted a data-based versioning format. See: https://github.com/ncihtan/data-models?tab=readme-ov-file#data-release-process

The HTAN 2.0 data model should adopt a semantic versioning approach. Suggest that we take an identical approach to MC2: https://mc2-center.github.io/data-models/contributing/

This update include updating our data model release documentation in the README of the new HTAN 2.0 data models repo and/or providing additional information within our HTAN data models documentation (https://ncihtan.github.io/data-models/)