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/utils/number-sidebar-items.mjs

@@ -5,7 +5,6 @@
  * @returns {import("@/types").SidebarItem[]} The modified sidebar items
  */
 export default function numberSidebarItems(sidebarItems, numbering = [1]) {
-  // TODO generate chapter titles
   if (!numbering.length) {
     numbering.push(1)
   }
@@ -24,7 +23,7 @@ export default function numberSidebarItems(sidebarItems, numbering = [1]) {
     item.chapterTitle = `${item.number} ${
       item.chapterTitle?.trim() || item.title?.trim()
     }`
-    item.title = `${item.number} ${item.title.trim()}`
+    item.title = item.title.trim()
 
     if (isTopItems) {
       // Add chapter category