Add NEMO and MITgcm test data to `datasets.structured.circulation_models`

[VeckoTheGecko]

• [x] Chose the correct base branch (v4-dev)
• [x] contributes to #2005
• [x] Added documentation

early WIP to get comment on data availability

The goal with these datasets is to mimick what the model output looks like (i.e., metadata (naming, units etc.), file layout, dimension ordering, variable naming, data types of variables, actual variables being provided) so that we can account for this in Parcels. These test datasets won't have realistic data - beyond scope of these datasets and this PR.

[VeckoTheGecko]

Back burner for now https://github.com/OceanParcels/Parcels/issues/2004#issuecomment-2948720414

[VeckoTheGecko]

Will close this PR as its superceded by #2053