docs: Update msrv badge and remove doc covered by it #1679

Job	Run time
test (stable)	35s
test (beta)	31s
test (nightly)	26s
check-external-types / check-external-types	20s
deny / deny	15s
miri / miri	37s
docs / docs	13s
msrv / msrv	4m 9s
tidy / tidy	50s
	7m 56s

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