tree: Add module-level docs #501

Job	Run time
build (bls, fedora-compat)	8m 42s
build (bls, fedora)	6m 57s
build (bls, arch)	7m 29s
build (uki, arch)	7m 16s
build (unified, fedora)	8m 52s
build (uki, fedora)	6m 43s
build (bls, rawhide)	6m 18s
build (bls, arch, xfs, none)	7m 3s
build (unified-secureboot, fedora)	7m 29s
build (bls, arch, ext4, none)	5m 3s
build (bls, rhel9)	7m 31s
	1h 19m 23s

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