Add example of generating a cert chain #507

Job	Run time
Measure coverage	8m 29s
Clippy	37s
build-windows	17m 3s
build (macOS-latest, stable)	8m 26s
Documentation (stable)	1m 1s
build (ubuntu-latest, stable)	12m 15s
Documentation (beta)	58s
Format	20s
Documentation (nightly)	54s
build (ubuntu-latest, beta)	55s
Documentation (stable 7 months ago)	1m 2s
build (ubuntu-latest, nightly)	10m 12s
build (ubuntu-latest, stable 7 months ago)	9m 21s
	1h 11m 33s

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