build(docs-infra): temporarily serve the DevTools guide at `/devtools` too (#42283)

The latest SW app version knows that `/devtools` should be redirected to
`/guide/devtools`. However, if a user has an older app version installed
(and until the SW downloads the latest version) they will get a 404 when
navigating to `/devtools`.

The reason is that the SW will see `/devtools` as a regular navigation
URL and serve `index.html` instead. The Angular app will then try to get
the content for `/devtools` by fetching `/generated/docs/devtools.json`
(which does not exist).

This commit fixes the issue by redirecting
`/generated/docs/devtools.json` to
`/geenrated/docs/guide/devtools.json`. When a user visits `/devtools`
for the first time, they will still be able to see the guide content
(while the SW updates in the background). On subsequent visits (once the
SW has downloaded the latest app version), they will be redirected to
`/guide/devtools`).

At a later time, once we are confident that the majority of users will
have updated to a newer app version, we can remove this temporary
redirect.

PR Close #42283

aio/firebase.json

@@ -136,7 +136,11 @@
       {"type": 301, "source": "/strict", "destination": "/guide/strict-mode"},
 
       // Use discoverable link for Angular DevTools and redirect to the guide page
-      {"type": 301, "source": "/devtools", "destination": "/guide/devtools"}
+      {"type": 301, "source": "/devtools", "destination": "/guide/devtools"},
+
+      // Temporarily serve the guide at `/devtools` as well as `/guide/devtools` until users have
+      // their SW updated to a version that knows about the `/devtools` redirect.
+      {"type": 302, "source": "/generated/docs/devtools.json", "destination": "/generated/docs/guide/devtools.json"}
     ],
     "rewrites": [
       {