Add docs/integrations/data-import.md covering the Apple Health
export.zip path, the generic JSON import schema with the full
MeasurementType enum and canonical units, a worked example, and a
CSV-to-JSON conversion how-to. Fix the Apple Health doc to point at
Settings -> Export & Import instead of a non-existent import page. Extend
the export e2e spec with the import surfaces and the example-download
happy path.
