CI: move `doc` job to workspace level (#1489) #2318

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	19s
build (1.71.0, wasm32-unknown-unknown)	18s
build (stable, thumbv7em-none-eabi)	10s
test (1.71.0)	24s
build (stable, wasm32-unknown-unknown)	11s
test (stable)	16s
	1m 38s

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