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/globals.css

@@ -16,10 +16,14 @@
     @apply bg-medusa-bg-highlight;
   }
 
-  * {
+  *:not(.code-block-elm) {
     scrollbar-color: var(--docs-border-base) var(--docs-bg-base);
   }
 
+  .code-block-elm {
+    scrollbar-color: var(--docs-contrast-border-base) transparent;
+  }
+
   aside * {
     scrollbar-color: var(--docs-border-base) var(--docs-bg-subtle);
   }