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/package.json

@@ -16,6 +16,7 @@
   "dependencies": {
     "@mdx-js/loader": "^3.0.0",
     "@mdx-js/react": "^3.0.0",
+    "@medusajs/icons": "rc",
     "@next/mdx": "^14.2.14",
     "clsx": "^2.1.0",
     "docs-ui": "*",