add(board): add Pimoroni Explorer RP2350 (PIM720) #17564

Job	Run time
scheduler	49s
docs	12m 58s
mpy-cross-mac	29s
mpy-cross / build (static-mingw)	48s
tests / run (mpy)	38s
tests / run (native_mpy)	41s
mpy-cross / build (static-aarch64)	44s
tests / zephyr	3m 30s
mpy-cross / build (static-raspbian)	45s
mpy-cross / build (static)	22s
tests / run (all)	1m 6s
tests / run (native)	54s
windows-zephyr	0s
windows	0s
ports (raspberrypi) / board (pimoroni_explorer2350)	2m 18s
	26m 2s

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