Update mdoc to 2.3.8 #1225

Job	Run time
examples (ubuntu-latest)	4m 59s
integration-tests (ubuntu-20.04)	38m 10s
examples (windows-latest)	8m 39s
bincompat	11m 45s
integration-tests (macos-latest)	47m 50s
format	21s
integration-tests (windows-latest)	36m 42s
publishLocal	27m 15s
test (ubuntu-latest, 8, 2.12.17)	3m 59s
website	2m 8s
test (ubuntu-latest, 8, 2.12.18)	3m 42s
test (ubuntu-latest, 8, 2.13.10)	3m 31s
test (ubuntu-latest, 8, 2.13.11)	4m 28s
test (ubuntu-latest, 8, 3.3.0)	4m 55s
test (windows-latest, 8, 2.13.11)	5m 59s
test (macos-latest, 8, 2.13.11)	5m 0s
test (ubuntu-latest, 17, 2.12.18)	4m 27s
all-tests	1s
	3h 33m 51s

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