docs: Remove or implement the atompack[hf] install extra

[speckhard]

atompack-py/python/atompack/hub.py:22-24 suggests users reinstall with:

  "reinstall atompack with 'atompack[hf]' or install 'huggingface_hub>=0.24'"

however:

• pyproject.toml does not define an hf extra, so
  pip install atompack-db[hf] is a no-op with a pip warning.
• huggingface_hub>=0.24 is already a required base dependency
  (pyproject.toml:35), so the ModuleNotFoundError branch is essentially
  unreachable in a healthy install anyway.