cmpv2

x509-cert: document `add_extension` link to examples (#1513) #1403

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	29s
build (1.75.0, wasm32-unknown-unknown)	28s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	24s
build (stable, wasm32-unknown-unknown)	30s
test (1.75.0)	3m 58s
test (stable)	3m 27s
	9m 16s

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