Discrepency in data_thumbnail placement between code and ivoa note

[olivialynn]

Fairly nitpicky but would be nice to address sometime. Tagging @nevencaplar for visibility, as this is a follow-up.

The IVOA note describes data_thumbnail as being directly in catalog/:

catalog/
|-- [REQUIRED] properties
|-- [RECOMMENDED] partition_info.csv
|-- [OPTIONAL] point_map.fits
|-- [OPTIONAL] data_thumbnail.parquet
+-- dataset/
    |-- [RECOMMENDED] _metadata
    |-- [RECOMMENDED] _common_metadata
    |-- Norder=0/
    |-- Norder=1/
    |-- Norder=2/
    |-- Norder=3/
    |-- Norder=4/
    |-- Norder=5/
    |-- Norder=6/
    +-- Norder=. . ./

However, our code handles it as being in catalog/dataset/:

The code to get_data_thumbnail_pointer:

    return get_upath(catalog_base_dir) / DATASET_DIR / DATA_THUMBNAIL_FILENAME

Also, in a docstring describing the metadata files:

        catalog/
        ├── ...
        └── dataset/
            ├── data_thumbnail.parquet    (only if create_thumbnail=True)
            ├── ...

No strong feelings on which is to be preferred (and so I wasn't sure what tag to assign—documentation, if we want to update the note? can we even do that?), I just think it would be neat if they matched.