Add very basic allergies documentation

To appease the scarf

doc/src/content/docs/en/mod/json/reference/items/allergies.md

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+---
+title: allergies
+---
+
+## Basic Info
+Allergies are based on the vitamins system for the purpose of inheritability and avoiding a need to create several variants of items / make tough calls as to the material of foods.
+
+As an initial measure, these vitamins were applied based on materials to prevent regression and having to add the vitamins all by hand. However, unless an item has `NUTRIENT_OVERRIDE`, cooking *should* correctly reflect what actually went into it. Naturally spawned items will have whatever their materials would indicate.
+
+If a food item has veggy allergens in it, a carnivore will be unable to eat it. Vice versa for an herbivore and something with meat allergens.
+
+## Current 'allergens':
+- `"egg_allergen"`
+- `"fruit_allergen"`
+- `"human_flesh_vitamin"` (This one is a special case, as it's instead used for morale maluses / boons due to Cannibalism)
+- `"junk_allergen"`
+- `"meat_allergen"`
+- `"milk_allergen"`
+- `"nut_allergen"`
+- `"veggy_allergen"`
+- `"wheat_allergen"` (Actually just general bread / baked goods)
+
+## Current dietary restrictions:
+Full bans:
+ - Carnivore:  veggy_allergen, fruit_allergen, wheat_allergen, nut_allergen
+ - Herbivore / Ruminant: meat_allergen, egg_allergen
+Dislikes:
+ - Vegetarian
+ - Anti-plant
+ - Lactose intolerance
+ - Fruit intolerance
+ - Junkfood intolerance
+ - Grain intolerance