docs: zero out narrative debt (Phase 2 L4)

docs(install): extract troubleshooting to its own file

docs(sprint-H.9): archive migration plans + cleanup stale doc paths

feat(sprint-H.5): cleanup baselines + tests obsolètes

docs(7.D): cleanup docs publiques + README + tests CLI restants

docs: refonte Diataxis + 8 documents institutionnels (S60)