Rename `doc_cfg` to `docsrs` and use `doc_auto_cfg` #836

Job	Run time
Check doc	1m 4s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	51s
test (macos-latest, x86_64-apple-darwin, stable)	48s
test (windows-latest, x86_64-pc-windows-gnu, stable)	1m 22s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 15s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 53s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	3m 1s
test-ios	19s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 45s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 33s
test-miri	3m 31s
test-no-std	11s
	17m 33s

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