Add `parcels._datasets` subpackage and developer documentation #804

Job	Run time
mypy	3m 36s
py3.13 \| macos \| integration tests	11m 30s
py3.13 \| macos \| unit tests	10m 50s
py3.13 \| ubuntu \| integration tests	13m 31s
py3.13 \| ubuntu \| unit tests	11m 4s
py3.13 \| windows \| integration tests	26m 19s
py3.13 \| windows \| unit tests	13m 59s
py3.11 \| ubuntu \| integration tests	21m 54s
py3.11 \| ubuntu \| unit tests	10m 58s
py3.12 \| ubuntu \| unit tests	11m 29s
merge-test-artifacts	0s
	2h 15m 10s

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