Change Currency, Upgrade docs, add Item docs

Change Upgrade docs to mdx, update type info.

documentation/docs/main/currency.md

@@ -21,6 +21,7 @@ To create a currency, do either of the following:
 
     // 1 parameter - `name` is required. It must be unique,
     // as it is used for saving/loading to identify the currnecy.
+    // Another parameter - `upgrades` is optional. See the `Upgrade` class for more information.
     const myCurrency = myGame.addCurrency("myCurrency");
 
     // All the methods are in `<GameCurrency>.static`, which contains the same methods as <CurrencyStatic>

documentation/docs/main/item.md

@@ -0,0 +1,38 @@
+---
+id: item
+title: Item
+sidebar_label: Item
+---
+
+[Link to auto generated docs](https://xshadowblade.github.io/emath.js/typedoc/classes/classes_Item.Item.html)
+
+The `Item` class represents an item that can be bought with a currency. An item is similar to an upgrade, except that it doesn't have a level nor a scalable cost.
+
+It can be used internally in the `CurrencyStatic` class.
+
+## Usage
+
+There is currently only one way to create items:
+
+1. Create items using the `addItem` method of the `CurrencyStatic` class. See [Currency Docs](./currency) for more information on the ways to create currencies.
+
+```js
+// If using `CurrencyStatic` class on its own (not recommended)
+// import { myCurrency } from "./currency.js";
+
+// If using `Game` class
+import { myGame } from "./game.js";
+// Create the currency with the game (See Currency docs for more information)
+const myCurrency = myGame.addCurrency("myCurrency");
+
+myCurrency.addItem({
+    id: "item1",
+    name: "Gold Coin",
+    description: "A coin made of gold.",
+    effect: (amount) => console.log(`You have ${amount} gold coins.`),
+    cost: () => new Decimal(1000),
+});
+
+// Buy the max amount of the item
+myCurrency.buyItem("item1");
+```

documentation/docs/main/main.md

@@ -12,5 +12,6 @@ This category lists the primary exports of the package `emath.js`.
 - [Boost](./boost)
 - [Currency](./currency)
 - [Grid](./grid)
+- [Item](./item)
 - [Numerical Analysis](./numerical-analysis)
 - [Upgrade](./upgrade)