enable `doc_cfg` feature in docsrs #12

Job	Run time
cargo-hack	18s
check-docs	13s
cargo-deny check (advisories)	23s
check	41s
Spell Check with Typos	8s
cargo-deny check (bans licenses sources)	25s
test-msrv	2m 6s
test-versions (stable)	23s
test-versions (beta)	42s
test-versions (nightly)	42s
	6m 1s

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