convnext refactoring

[chapman20j]

Simplifies the ConvNeXT implementation and adds 3 configs to the model config.
Added additional tests for the other configs.
Added readme for this model.

Checklist

• I have read the Contribution Guidelines and used pre-commit hooks to format this commit.
• I have added all the necessary unit tests for my change. (run_model.py for model usage, test_outputs.py and/or model_validation_colab.ipynb for quality).
• (If using an LLM) I have carefully reviewed and removed all superfluous comments or unneeded, commented-out code. Only necessary and functional code remains.
• I have signed the Contributor License Agreement (CLA).