Clarify OER_DATASET_PATH and contents

The docs are a bit vague on this in `FinetuneOlmoEarthSegmentation` stating
```
export OER_DATASET_PATH=/path/to/your/oerun_dataset/folder # Replace with desired dataset folder path
```
I created a new folder and used that, then `prepare_labeled_windows` fails with
```
ValueError: Annotation task features file not found: /teamspace/studios/this_studio/data/oerun_dataset/annotation_task_features.geojson
```
However I see that file has been created at some point at
```
olmoearth_projects/docs/tutorials/FinetuneOlmoEarthSegmentation/config/annotation_task_features.geojson
```
So assume this needs to be copied over to `oerun_dataset`