Add `parcels._datasets` subpackage and developer documentation #805

Job	Run time
mypy	3m 51s
py3.13 \| macos \| unit tests	10m 51s
py3.13 \| macos \| integration tests	11m 56s
py3.13 \| ubuntu \| unit tests	11m 41s
py3.13 \| ubuntu \| integration tests	14m 32s
py3.13 \| windows \| unit tests	14m 33s
py3.13 \| windows \| integration tests	14m 56s
py3.11 \| ubuntu \| unit tests	11m 31s
py3.11 \| ubuntu \| integration tests	14m 28s
py3.12 \| ubuntu \| unit tests	12m 2s
merge-test-artifacts	1s
	2h 0m 22s

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