CI: add `cargo doc` #681

Job	Run time
check format	6s
doc	15s
check	13s
msrv	35s
check (--no-default-features)	7s
test	54s
check (--no-default-features --features use_alloc)	7s
check (--all-targets --all-features)	22s
All checks succeeded	1s
	2m 40s

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