split data in separate files

docs/population-tiers/tier-i.md

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+# Tier I - Liberti (Latium) / Waders (Albion)
+
+**Entity Type:** PopulationTier
+
+## Stats
+
+| Field | Value |
+|-------|-------|
+| Workforce Fraction | 50% |
+| Needs Categories | Food, Public Service, Fashion |
+
+## Notes
+
+Avoid upgrading all Tier I housing at once; you can lose Tier I workers needed for basic chains.
+
+## Common Pitfalls
+
+"Everything upgraded" leads to fisheries/farms/sawmills being understaffed.
+
+## Related
+
+- [Sardines](../goods/sardines.md) - Food need (Latium)
+- [Porridge](../goods/porridge.md) - Food need
+- [Tunics](../goods/tunics.md) - Fashion need