Configure docs for feature gates globally #579

Job	Run time
Pass MSRV values to other jobs	4s
nostd/thumbv6m-none-eabi/stable	16s
clippy/stable	22s
miri	11m 6s
docs/nightly	21s
cross_test/s390x-unknown-linux-gnu/stable	8m 14s
cross_test/i686-unknown-linux-gnu/stable	1m 53s
format/nightly	16s
cargo-careful	1m 39s
blas-msrv	40s
tests/nightly	2m 34s
tests/stable	2m 27s
tests/beta	2m 17s
tests/1.64.0	3m 21s
conclusion	2s
	35m 32s

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