Add `parcels._datasets` subpackage and developer documentation #803

Job	Run time
mypy	1m 30s
py3.13 \| macos \| integration tests	1m 33s
py3.13 \| macos \| unit tests	1m 32s
py3.13 \| ubuntu \| integration tests	1m 30s
py3.13 \| ubuntu \| unit tests	1m 31s
py3.13 \| windows \| integration tests	1m 29s
py3.13 \| windows \| unit tests	1m 29s
py3.11 \| ubuntu \| integration tests	1m 31s
py3.11 \| ubuntu \| unit tests	1m 42s
py3.12 \| ubuntu \| unit tests	1m 43s
merge-test-artifacts	1s
	15m 31s

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