multiboot2-common: doc improvement with figures #999

Job	Run time
build (stable) / rust	12s
build (nightly) / rust	29s
build (msrv) / rust	20s
build no_std (stable) [Windows] / rust	38s
build no_std (stable) [w/o builder] / rust	26s
style (stable) / rust	21s
build no_std (stable) / rust	14s
style (msrv) / rust	23s
build no_std (msrv) / rust	25s
style (nightly) / rust	34s
build no_std (nightly) / rust	1s
tests with miri (nightly) / rust	1m 20s
	5m 23s

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