x509-cert: document `add_extension` link to examples (#1513) #984

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	30s
test (1.75.0)	28s
build (1.75.0, wasm32-unknown-unknown)	23s
minimal-versions	0s
test (stable)	25s
build (stable, thumbv7em-none-eabi)	24s
build (stable, wasm32-unknown-unknown)	46s
	2m 56s

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