Add `parcels._datasets` subpackage and developer documentation #800

Job	Run time
mypy	3m 51s
py3.13 \| macos \| unit tests	9m 20s
py3.13 \| macos \| integration tests	11m 23s
py3.13 \| ubuntu \| unit tests	13m 28s
py3.13 \| ubuntu \| integration tests	14m 7s
py3.13 \| windows \| unit tests	14m 26s
py3.13 \| windows \| integration tests	24m 1s
py3.11 \| ubuntu \| unit tests	12m 8s
py3.11 \| ubuntu \| integration tests	35m 21s
py3.12 \| ubuntu \| unit tests	12m 20s
merge-test-artifacts	0s
	2h 30m 25s

Provide feedback