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docs: three trees, three audiences, and the rule for citing between them #1512

Job	Run time
e2e 2/2 (linux x86_64, self-host)	8m 56s
bare-metal e2e (riscv64-none-elf + cortex-m, qemu)	5m 52s
e2e 1/2 (linux x86_64, self-host)	8m 55s
hermetic e2e (no host toolchain, container)	6m 25s
	30m 8s