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/packages/docs-ui/src/components/IconHeadline/index.tsx

@@ -0,0 +1,15 @@
+import React from "react"
+
+type IconHeadlineProps = {
+  title: string
+  icon: React.ReactNode
+}
+
+export const IconHeadline = ({ title, icon }: IconHeadlineProps) => {
+  return (
+    <div className="flex gap-docs_0.5 text-medusa-fg-base">
+      {icon}
+      <span className="text-small-plus">{title}</span>
+    </div>
+  )
+}