ModelConfig.tokenizer_path and TrainConfig.save_best

### 📚 The doc issue

ModelConfig.tokenizer_path has perhaps moved to something like TokenizerConfig? It is under `tokenizer:` block in the yml config files. Also, `save_best` under TrainConfig is not documented in the readthedocs.

### Suggest a potential alternative/fix

_No response_

Locate the definitions or documentation for ModelConfig.tokenizer_path, TokenizerConfig, and TrainConfig.save_best, then compare them with the `tokenizer:` YAML configuration block. Update the relevant ReadTheDocs pages so both settings are accurately described, and verify the rendered documentation matches the current configuration names.

Written by the indexing model from the issue text.