docs: added loaders documentation (#4019)

* docs: added loaders documentation

* added a link to the loaders documentation

docs/content/development/loaders/create.md

@@ -0,0 +1,86 @@
+---
+description: 'Learn how to create a loader in Medusa. A loader can be created in the Medusa backend codebase, in a plugin, or in a module.'
+addHowToData: true
+---
+
+# How to Create a Loader
+
+In this document, you’ll learn how to create a loader in Medusa. A loader can be created in the Medusa backend codebase, in a plugin, or in a module.
+
+## Step 1: Create Loader File
+
+Create a TypeScript or JavaScript file in the `src/loaders` directory that will hold your custom script. There are no restrictions on the name of the file.
+
+For example, create the file `src/loaders/my-loader.ts` that will hold the loader.
+
+---
+
+## Step 2: Define the Loader
+
+The loader file must export a function.
+
+### Parameters of Loaders in Medusa Backend and Plugins
+
+When the loader is defined in the Medusa backend or a plugin, the function receives the following parameters:
+
+1. `container`: the first parameter, which is a `AwilixContainer` object. You can use the container to register custom resources into the dependency container or resolve resources from the dependency container.
+2. `config`: the second parameter, which is an object that holds the loader’s plugin options. If the loader is not created inside a plugin, the config object will be empty.
+
+### Parameters of Loaders in Modules
+
+When the loader is defined in a module, it receives the following parameters:
+
+1. `container`: the first parameter, which is a `AwilixContainer` object. You can use the container to register custom resources into the dependency container or resolve resources from the dependency container.
+2. `logger`: the second parameter, which is a `Logger` object. The logger can be used to log messages in the console.
+3. `config`: the third parameter, which is an object that holds the loader’s module options.
+
+### Example Implementation
+
+For example, this loader function resolves the `ProductService` and logs in the console the count of products in the Medusa backend:
+
+```ts title=src/loaders/my-loader.ts
+import { ProductService } from "@medusajs/medusa"
+import { AwilixContainer } from "awilix"
+
+export default async (
+  container: AwilixContainer,
+  config: Record<string, unknown>
+): Promise<void> => {
+  console.info("Starting loader...")
+  const productService = container.resolve<ProductService>(
+    "productService"
+  )
+  console.info(`Products count: ${
+    await productService.count()
+  }`)
+  console.info("Ending loader...")
+}
+```
+
+---
+
+## Step 3: Run Build Command
+
+In the directory of your project, run the following command to transpile the files from the `src` to the `dist` directory:
+
+```bash npm2yarn
+npm run build
+```
+
+---
+
+## Test it Out
+
+:::note
+
+This section explains how to test out the loader if it’s created in the Medusa backend codebase. If you’re creating your loader in a plugin, you can learn how to test it in the [plugins documentation](../plugins/create.md#test-your-plugin). Alternatively, if you’re creating your loader in a module, you can learn how to test it in the [modules documentation](../modules/create.mdx#step-4-test-your-module).
+
+:::
+
+Run the following command to start the Medusa backend:
+
+```bash npm2yarn
+npm run start
+```
+
+Your loader script should run on the Medusa backend startup. If you logged a message in the console, similar to the example above, you should see it in the console.