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x509-cert: document `add_extension` link to examples (#1513) #3102

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	35s
minimal-versions	0s
build (1.72.0, wasm32-unknown-unknown)	48s
build (stable, thumbv7em-none-eabi)	40s
test (1.72.0)	1m 12s
build (stable, wasm32-unknown-unknown)	38s
test (stable)	1m 15s
	5m 8s

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