docs: Update msrv badge and remove doc covered by it #2098

Job	Run time
test (1.37)	1m 13s
test (stable)	29s
test (beta)	40s
test (nightly)	1m 35s
check-external-types / check-external-types	13s
deny / deny	15s
docs / docs	14s
miri / miri	1m 0s
msrv / msrv	6m 26s
tidy / tidy	1m 9s
	13m 14s

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