Migrate to `doc_auto_cfg` #1295

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 19s
build (1.65.0, wasm32-unknown-unknown)	5m 26s
test (1.65.0)	4m 37s
build (stable, thumbv7em-none-eabi)	22s
test (stable)	25s
build (stable, wasm32-unknown-unknown)	16s
minimal-versions / minimal-versions	24s
	16m 49s

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