Add information about patterns search order to the doc about structuring repo
Following [this](https://github.com/huggingface/datasets/issues/5650) issue I think we should add a note about the order of patterns that is used to find splits, see [my comment](https://github.com/huggingface/datasets/issues/5650#issuecomment-1488412527). Also we should reference this page in pages about packaged loaders. 

I have a déjà vu that it had already been discussed as some point but I don't remember....