Move `data_thumbnail.parquet` out of the dataset directory

[hombit]

Feature request

It would be convinent to consider dataset/ directory as a consistent parquet dataset, with hive partitioning and no data duplication. Having some extra data there may cause some problems with parquet-dataset readers which are not aware of HATS, e.g. Hugging Face datasets library, see #615 for the context.

P.S. Actually HATS standard section 3.1 says that this file should be in the catalog root:
https://www.ivoa.net/documents/Notes/HATS/20250822/NOTE-hats-ivoa-1.0-20250822.html

[nevencaplar]

We have just actually patched the note so that it is consistent with what the data and the code were doing (#614). We can switch it back if it is needed.

[hombit]

I believe it should not be in the dataset, for the reasons I described above: this will cause data duplication for users opening the dataset with non-HATS-aware Parquet-dataset readers.