Migrate to `doc_auto_cfg` (#1370) #2053

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	4m 43s
build (1.56.0, wasm32-unknown-unknown)	5m 32s
build (stable, thumbv7em-none-eabi)	19s
build (stable, wasm32-unknown-unknown)	23s
derive (1.56.0)	4m 37s
derive (stable)	14s
doc	31s
test (1.56.0)	5m 56s
test (stable)	23s
minimal-versions / minimal-versions	42s
	23m 20s

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