add some basic crate docs (#55) #190

Job	Run time
Check Style	13s
Test with Miri	43s
Check MSRV (1.63)	4m 2s
features	39s
Test stable on ubuntu-latest	24s
Test stable on windows-latest	50s
Test stable on macOS-latest	1m 3s
Test beta on ubuntu-latest	24s
Test beta on windows-latest	54s
Test beta on macOS-latest	1m 11s
Test nightly on ubuntu-latest	22s
Test nightly on windows-latest	59s
Test nightly on macOS-latest	1m 23s
Build docs	19s
CI is green	1s
	13m 27s

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