Update mdoc to 2.6.1 (#1420) #1599

Job	Run time
bincompat	3m 3s
integration-tests (ubuntu-22.04)	29m 26s
examples (ubuntu-latest)	2m 33s
integration-tests (macos-12)	37m 56s
format	25s
examples (windows-latest)	4m 6s
publishLocal	16m 39s
integration-tests (windows-latest)	23m 37s
test (ubuntu-latest, 8, 2.12.18)	2m 32s
website	1m 15s
test (ubuntu-latest, 8, 2.12.19)	2m 19s
test (ubuntu-latest, 8, 2.13.13)	2m 37s
test (ubuntu-latest, 8, 2.13.14)	2m 26s
test (ubuntu-latest, 8, 3.3.3)	2m 21s
test (ubuntu-latest, 8, 3.4.2)	2m 30s
test (windows-latest, 8, 2.13.14)	3m 5s
test (macos-12, 8, 2.13.14)	2m 29s
test (ubuntu-latest, 17, 2.13.14)	2m 40s
all-tests	1s
	2h 22m 0s

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