multiboot2-common: improve README and diagrams #1072

Job	Run time
build (stable) / rust	13s
build (msrv) / rust	18s
build (nightly) / rust	25s
style (stable) / rust	57s
build no_std (stable) [Windows] / rust	1m 7s
build no_std (stable) / rust	17s
build no_std (stable) [w/o builder] / rust	27s
build no_std (msrv) / rust	1m 5s
style (msrv) / rust	21s
style (nightly) / rust	28s
build no_std (nightly) / rust	2m 4s
tests with miri (nightly) / rust	1m 43s
	9m 25s

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