NationWars

Added first-stage datapack support for doctrine customization.

New
  - Existing doctrines can now be overridden with datapack JSON files.
  - Server owners can edit doctrine display names, icons, ideology, free claims, capture time, claim cost, income, maintenance, market multipliers, peace cost, surrender land multiplier, supported mechanic toggles, and menu lore.
  - Doctrine icons can now use item IDs from JSON instead of only the built-in hardcoded blocks.
  - Added docs with the datapack folder path and a full example JSON file.

Notes
  - This first pass supports editing the existing doctrine IDs only: german, soviet, american, french, british, italian, and romanian.
  - Fully custom new doctrines and brand new custom perk logic are planned as a later stage because save data, nation creation, and some country-specific perk behavior still depend on the current doctrine IDs.