docs: add routing page (#9550)

- Add a new homepage to `book` project for the routing page
- Move all main doc pages to be under `/v2/learn` (and added redirects + fixed links across docs)
- Other: add admin components to resources dropdown + fixes to search on mobile.

Closes DX-955

Preview: https://docs-v2-git-docs-router-page-medusajs.vercel.app/v2

www/apps/book/app/learn/basics/commerce-modules/page.mdx

@@ -0,0 +1,55 @@
+export const metadata = {
+  title: `${pageNumber} Commerce Modules`,
+}
+
+# {metadata.title}
+
+In this chapter, you'll learn about Medusa's commerce modules.
+
+## What is a Commerce Module?
+
+Medusa provides all its commerce features as separate commerce modules, such as the Product or Order modules. Medusa uses these modules in its API routes to expose their commerce features.
+
+Medusa's commerce modules and your custom modules are interchangeable in the Medusa application, making Medusa’s architecture more flexible.
+
+### List of Medusa's Commerce Modules
+
+Refer to [this reference](!resources!/commerce-modules) for a full list of commerce modules in Medusa.
+
+---
+
+## Resolve Commerce Module Services
+
+Similarly to your custom module, a commerce module's main service is registered in the Medusa container. So, you can resolve it in your resources, such as API routes, to use its functionality.
+
+For example, you saw this code snippet in the [Medusa container chapter](../medusa-container/page.mdx):
+
+```ts highlights={[["10"]]}
+import type { MedusaRequest, MedusaResponse } from "@medusajs/framework/http"
+import { IProductModuleService } from "@medusajs/framework/types"
+import { Modules } from "@medusajs/framework/utils"
+
+export const GET = async (
+  req: MedusaRequest, 
+  res: MedusaResponse
+) => {
+  const productModuleService: IProductModuleService = req.scope.resolve(
+    Modules.PRODUCT
+  )
+
+  const [, count] = await productModuleService
+    .listAndCountProducts()
+
+  res.json({
+    count,
+  })
+}
+```
+
+When you resolve the `Modules.PRODUCT` (or `productModuleService`) registration name, you're actually resolving the main service of the Product Module.
+
+<Note title="Tip">
+
+To resolve the main service of any commerce module, use the registration name defined in the `Modules` enum imported from `@medusajs/framework/utils`.
+
+</Note>