ExtendedTask: add example to docs #2654

Job	Run time
website / pkgdown	2m 26s
routine / routine	4m 43s
R-CMD-check / setup	6s
R-CMD-check / macOS-latest (4.4.2)	3m 11s
R-CMD-check / windows-latest (4.4.2)	4m 14s
R-CMD-check / ubuntu-latest (devel)	3m 18s
R-CMD-check / ubuntu-latest (4.4.2)	3m 21s
R-CMD-check / ubuntu-latest (4.3.3)	3m 20s
R-CMD-check / ubuntu-latest (4.2.3)	3m 31s
R-CMD-check / ubuntu-latest (4.1.3)	3m 28s
R-CMD-check / ubuntu-latest (4.0.5)	3m 20s
	34m 58s

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