docs: improvements + additions to module docs (#9152)

- Split Module and Module Links to their own chapters
- Add new docs on db operations and transactions in modules, multiple services, links with custom columns, etc...
- Added a list of registered dependencies in a module container

www/apps/book/app/advanced-development/modules/isolation/page.mdx

@@ -8,8 +8,8 @@ In this chapter, you'll learn how modules are isolated, and what that means for
 
 <Note title="Summary">
 
-- Modules can't access resources, such as services, from other modules.
-- You can use Medusa's tools, as explained in the next chapters, to extend a modules' features or implement features across modules.
+- Modules can't access resources, such as services or data models, from other modules.
+- Use Medusa's linking concepts, as explained in the [Module Links chapters](../../module-links/page.mdx), to extend a module's data models and retrieve data across modules.
 
 </Note>
 
@@ -21,12 +21,93 @@ For example, your custom module can't resolve the Product Module's main service
 
 ---
 
-## How to Implement Custom Features Across Modules?
+## Why are Modules Isolated
 
-In your Medusa application, you want to implement features that span across modules, or you want to extend an existing module's features and customize them for your own use case.
+Some of the module isolation's benefits include:
 
-For example, you want to extend the Product Module to add new properties to the `Product` data model.
+- Integrate your module into any Medusa application without side-effects to your setup.
+- Replace existing modules with your custom implementation, if your use case is drastically different.
+- Use modules in other environments, such as Edge functions and Next.js apps.
 
-Medusa provides the tools to implement these use cases while maintaining isolation between modules.
+---
 
-The next chapters explain these tools and how to use them in your custom development.
+## How to Extend Data Model of Another Module?
+
+To extend the data model of another module, such as the `product` data model of the Product Module, use Medusa's linking concepts as explained in the [Module Links chapters](../../module-links/page.mdx).
+
+---
+
+## How to Use Services of Other Modules?
+
+If you're building a feature that uses functionalities from different modules, use a workflow whose steps resolve the modules' services to perform these functionalities.
+
+Workflows ensure data consistency through their roll-back mechanism and tracking of each execution's status, steps, input, and output.
+
+### Example
+
+For example, consider you have two modules:
+
+1. A module that stores and manages brands in your application.
+2. A module that integrates a third-party Content Management System (CMS).
+
+To sync brands from your application to the third-party system, create the following steps:
+
+export const stepsHighlights = [
+  ["1", "retrieveBrandsStep", "A step that retrieves brands using a brand module."],
+  ["14", "createBrandsInCmsStep", "A step that creates brands using a CMS module."],
+  ["25", "", "Add a compensation function to the step if an error occurs."]
+]
+
+```ts title="Example Steps" highlights={stepsHighlights}
+const retrieveBrandsStep = createStep(
+  "retrieve-brands",
+  async (_, { container }) => {
+    const brandModuleService = container.resolve(
+      "brandModuleService"
+    )
+
+    const brands = await brandModuleService.listBrands()
+
+    return new StepResponse(brands)
+  }
+)
+
+const createBrandsInCmsStep = createStep(
+  "create-brands-in-cms",
+  async ({ brands }, { container }) => {
+    const cmsModuleService = container.resolve(
+      "cmsModuleService"
+    )
+
+    const cmsBrands = await cmsModuleService.createBrands(brands)
+
+    return new StepResponse(cmsBrands, cmsBrands)
+  },
+  async (brands, { container }) => {
+    const cmsModuleService = container.resolve(
+      "cmsModuleService"
+    )
+
+    await cmsModuleService.deleteBrands(
+      brands.map((brand) => brand.id)
+    )
+  }
+)
+```
+
+The `retrieveBrandsStep` retrieves the brands from a brand module, and the `createBrandsInCmsStep` creates the brands in a third-party system using a CMS module.
+
+Then, create the following workflow that uses these steps:
+
+```ts title="Example Workflow"
+export const syncBrandsWorkflow = createWorkflow(
+  "sync-brands",
+  () => {
+    const brands = retrieveBrandsStep()
+
+    updateBrandsInCmsStep({ brands })
+  }
+)
+```
+
+You can then use this workflow in an API route, scheduled job, or other resources that use this functionality.