chore: reorganize docs apps (#7228)

* reorganize docs apps

* add README

* fix directory

* add condition for old docs

www/apps/resources/app/commerce-modules/inventory/concepts/page.mdx

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+export const metadata = {
+  title: `Inventory Concepts`,
+}
+
+# {metadata.title}
+
+In this document, you’ll learn about the main concepts in the Inventory Module, and how data is stored and related.
+
+## InventoryItem
+
+An inventory item, represented by the `InventoryItem` data model, is a stock-kept item, such as a product, whose inventory can be managed.
+
+The `InventoryItem` data model mainly holds details related to the underlying stock item, but has relations to other data models that include its inventory details.
+
+![A diagram showcasing the relation between data models in the Inventory Module](https://res.cloudinary.com/dza7lstvk/image/upload/v1709658103/Medusa%20Resources/inventory-architecture_kxr2ql.png)
+
+---
+
+## InventoryLevel
+
+An inventory level, represented by the `InventoryLevel` data model, holds the inventory and quantity details of an inventory item in a specific location.
+
+It has three quantity-related fields:
+
+- `stocked_quantity`: The available stock quantity of an item in the associated location.
+- `reserved_quantity`: The quantity reserved from the available `stocked_quantity`. It indicates the quantity that's still not removed from stock, but considered as unavailable when checking whether an item is in stock.
+- `incoming_quantity`: The incoming stock quantity of an item into the associated location. This field doesn't play into the `stocked_quantity` or when checking whether an item is in stock.
+
+### Associated Location
+
+The inventory level's location is determined by the `location_id` field. The Medusa application links the `InventoryLevel` data model with the `StockLocation` data model from the Stock Location Module.
+
+Learn more about this relation in [this guide](../relations-to-other-modules/page.mdx#stock-location-module)
+
+---
+
+## ReservationItem
+
+A reservation item, represented by the `ReservationItem` data model, represents unavailable quantity of an inventory item in a location. It's used when an order is placed but not fulfilled yet.
+
+The reserved quantity is associated with a location, so it has a similar relation to that of the `InventoryLevel` with the Stock Location Module.