improve doc for new contributors #2364

Job	Run time
duplicates	2s
build (gcc-13.deb, --mini-tests)	6m 35s
build (gcc-13.deb, --std-tests)	6m 34s
build (gcc-13.deb, --test-libcore)	13m 36s
build (gcc-13.deb, --extended-rand-tests)	9m 26s
build (gcc-13.deb, --extended-regex-example-tests)	7m 30s
build (gcc-13.deb, --extended-regex-tests)	12m 14s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	16m 52s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	16m 56s
build (gcc-13.deb, --projects)	11m 59s
build (gcc-13-without-int128.deb, --mini-tests)	6m 38s
build (gcc-13-without-int128.deb, --std-tests)	6m 39s
build (gcc-13-without-int128.deb, --test-libcore)	13m 47s
build (gcc-13-without-int128.deb, --extended-rand-tests)	9m 35s
build (gcc-13-without-int128.deb, --extended-regex-example-tests)	7m 20s
build (gcc-13-without-int128.deb, --extended-regex-tests)	13m 3s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	16m 41s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	16m 48s
build (gcc-13-without-int128.deb, --projects)	12m 3s
build_system	49s
	3h 25m 7s

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