Document YAML configuration with "data_dir"
In particular, I would like to have an example of how to replace the following configuration (from https://huggingface.co/docs/hub/datasets-manual-configuration#splits)

```
---
configs:
- config_name: default
  data_files:
  - split: train
    path: "data/*.csv"
  - split: test
    path: "holdout/*.csv"
---
```

with the `data_dir` field.
See https://huggingface.co/datasets/uonlp/CulturaX/discussions/15#6597e83f185db94370d6bf50 for reference