ci: add documentation checks #4793

Job	Run time
arches	1s
mir-libs (amd64)	17m 55s
mir-libs (armhf)	28m 48s
mir-libs (arm64)	28m 9s
snap (canonical/mir-test-tools, amd64)	14m 12s
snap (canonical/mir-test-tools, armhf)	19m 45s
snap (canonical/mir-test-tools, arm64)	22m 51s
snap (canonical/confined-shell-wip, amd64)	6m 59s
snap (canonical/confined-shell-wip, armhf)	14m 53s
snap (canonical/confined-shell-wip, arm64)	13m 13s
snap (canonical/ubuntu-frame, amd64)	7m 57s
snap (canonical/ubuntu-frame, armhf)	12m 15s
snap (canonical/ubuntu-frame, arm64)	15m 16s
snap (Miriway/Miriway, amd64)	8m 22s
snap (Miriway/Miriway, armhf)	15m 3s
snap (Miriway/Miriway, arm64)	12m 59s
	3h 58m 38s

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