docs: new troubleshooting guide + cors fix (#6387)

- Fixed CORS middleware to account for routes of a single path (for example, `/custom` and not just `/custom/test`).
- Added a troubleshooting guide for admin webpack errors (see issue #6375)

www/apps/docs/content/development/api-routes/create.mdx

@@ -128,7 +128,7 @@ import cors from "cors"
 export const config: MiddlewaresConfig = {
   routes: [
     {
-      matcher: "/custom/*",
+      matcher: "/custom*",
       middlewares: [
         cors({
           origin: "*",