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/admin-customizations/page.mdx

@@ -0,0 +1,64 @@
+export const metadata = {
+  title: `${pageNumber} Admin Customizations`,
+}
+
+# {metadata.title}
+
+In this chapter, you’ll learn how to customize the Medusa Admin dashboard.
+
+## What is the Medusa Admin?
+
+The Medusa Admin is an admin dashboard that merchants use to manage their store's data.
+
+You can extend the Medusa Admin to add widgets and new pages. In your customizations, you interact with API routes to provide merchants with custom functionalities.
+
+The Medusa Admin is installed in your Medusa application and runs at `http://localhost:9000/app` when you start the application.
+
+---
+
+## Example: Create a Widget
+
+A widget is a React component that can be injected into an existing page in the admin dashboard.
+
+For example, create the file `src/admin/widgets/product-widget.tsx` with the following content:
+
+```tsx title="src/admin/widgets/product-widget.tsx"
+import { defineWidgetConfig } from "@medusajs/admin-sdk"
+import { Container, Heading } from "@medusajs/ui"
+
+const ProductWidget = () => {
+  return (
+    <Container className="divide-y p-0">
+      <div className="flex items-center justify-between px-6 py-4">
+        <Heading level="h2">Product Widget</Heading>
+      </div>
+    </Container>
+  )
+}
+
+export const config = defineWidgetConfig({
+  zone: "product.details.before",
+})
+
+export default ProductWidget
+```
+
+This inserts a widget with the text “Product Widget” at the beginning of a product’s details page.
+
+In your widget, use custom components from the [Medusa UI package](https://docs.medusajs.com/ui).
+
+### Test the Widget
+
+To test out the widget, start the Medusa application:
+
+```bash npm2yarn
+npm run dev
+```
+
+Then, open a product’s details page in the Medusa Admin. You’ll find your custom widget at the top of the page.
+
+---
+
+## Admin Components List
+
+To build admin customizations that match the Medusa Admin's designs and layouts, refer to [this guide](!resources!/admin-component) to find common components.