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/Breadcrumbs/index.tsx

@@ -93,7 +93,7 @@ export const Breadcrumbs = () => {
       className={clsx(
         "flex items-center gap-docs_0.25",
         "text-medusa-fg-muted text-compact-small",
-        "mb-docs_1"
+        "mb-docs_1 flex-wrap"
       )}
     >
       {Array.from(breadcrumbItems).map(([link, title], index) => (