docs(lib): document library stability (#3360) #2226

Job	Run time
Check Style	9s
Verify hyper.h is up to date	27s
Check MSRV	37s
features	32s
Test C API (FFI)	35s
Test with Miri	59s
udeps	42s
Test stable on ubuntu-latest	1m 34s
Test stable on windows-latest	1m 48s
Test stable on macOS-latest	1m 39s
Test beta on ubuntu-latest	1m 39s
Test beta on windows-latest	1m 47s
Test beta on macOS-latest	1m 39s
Test nightly on ubuntu-latest	1m 37s
Test nightly on windows-latest	1m 49s
Test nightly on macOS-latest	2m 1s
Build docs	0s
Check exposed types	0s
CI is green	0s
	19m 34s

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