docs: added loaders documentation (#4019)

* docs: added loaders documentation

* added a link to the loaders documentation

docs/content/development/loaders/create.md

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+---
+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.

docs/content/development/loaders/overview.mdx

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+---
+description: "earn what loaders are in Medusa. A loader is a script that runs when the Medusa backend starts."
+---
+
+import DocCard from '@theme/DocCard';
+import Icons from '@theme/Icon';
+
+# Loaders
+
+In this document, you’ll learn what loaders are in Medusa.
+
+## Overview
+
+A loader is a script that runs when the Medusa backend starts. The Medusa backend uses loaders to initialize the database connection, load plugins, register resources in the dependency container, and more.
+
+Loaders can be created within the Medusa backend codebase, in a plugin, or in a module to perform custom actions when the backend starts. The loader is created in a TypeScript or JavaScript file located in the `src/loaders` directory, then transpiled using the `build` command into the `dist/loaders` directory.
+
+### Loader Examples
+
+For example, the Redis Event Bus module uses a loader to establish a connection with Redis and log a message in the console.
+
+Another example is the Algolia plugin, which uses a loader to update the index settings when the Medusa backend starts based on the plugin’s options.
+
+Loaders can be used to access the dependency container and register custom resources in it.
+
+Loaders can also be used to create [scheduled jobs](../scheduled-jobs/overview.mdx) that run at a specified interval of time. For example:
+
+```ts
+const publishJob = async (container, options) => {
+  const jobSchedulerService = 
+    container.resolve("jobSchedulerService")
+  jobSchedulerService.create(
+    "publish-products", 
+    {}, 
+    "0 0 * * *", 
+    async () => {
+      // job to execute
+      const productService = container.resolve("productService")
+      const draftProducts = await productService.list({
+        status: "draft",
+      })
+
+      for (const product of draftProducts) {
+        await productService.update(product.id, {
+          status: "published",
+        })
+      }
+    }
+  )
+}
+
+export default publishJob
+```
+
+---
+
+## Custom Development
+
+Developers can create a loader with the desired functionality directly within the Medusa Core, in a plugin, or in a module.
+
+<DocCard item={{
+  type: 'link',
+  href: '/development/loaders/create',
+  label: 'Create a Loader',
+  customProps: {
+    icon: Icons['academic-cap-solid'],
+    description: 'Learn how to create a loader.'
+  }
+}} />