Add `parcels._datasets` subpackage and developer documentation #807

Job	Run time
mypy	3m 37s
py3.13 \| macos \| integration tests	10m 51s
py3.13 \| macos \| unit tests	9m 35s
py3.13 \| ubuntu \| integration tests	13m 24s
py3.13 \| ubuntu \| unit tests	11m 26s
py3.13 \| windows \| integration tests	24m 53s
py3.13 \| windows \| unit tests	14m 58s
py3.11 \| ubuntu \| integration tests	18m 49s
py3.11 \| ubuntu \| unit tests	12m 6s
py3.12 \| ubuntu \| unit tests	11m 31s
merge-test-artifacts	2s
	2h 11m 12s

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