Migrate to `doc_auto_cfg` #760

Job	Run time
build (1.61.0, thumbv7em-none-eabi)	4m 39s
build (1.61.0, wasm32-unknown-unknown)	5m 31s
build (stable, thumbv7em-none-eabi)	18s
test-msrv	4m 34s
test (1.61.0)	6m 16s
build (stable, wasm32-unknown-unknown)	22s
test (stable)	1m 7s
	22m 47s

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