kem

Migrate to `doc_auto_cfg` (#1370) #761

Job	Run time
build (1.61.0, thumbv7em-none-eabi)	4m 35s
test (1.61.0)	6m 32s
build (1.61.0, wasm32-unknown-unknown)	4m 38s
test (stable)	1m 19s
build (stable, thumbv7em-none-eabi)	22s
test-msrv	5m 2s
build (stable, wasm32-unknown-unknown)	31s
	22m 59s

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