docs: add a short redirect for HMR information

The `https://angular.dev/hmr` URL will now redirect
to the HMR information at the longer URL:
https://angular.dev/tools/cli/build-system-migration#hot-module-replacement
This shorter URL will be used within console messages within the Angular CLI.

Author: clydin

adev/src/app/routes.ts

@@ -223,6 +223,10 @@ const REDIRECT_ROUTES: Route[] = [
     path: 'guide/prerendering',
     redirectTo: '/guide/ssr',
   },
+  {
+    path: 'hmr',
+    redirectTo: '/tools/cli/build-system-migration#hot-module-replacement',
+  },
   {
     path: 'guide',
     children: [