HTML5Rocks is not deployed automatically.
If you make a change, you'll need to deploy it.
You can just run
gcloud from this folder (there's no build step):
$ gcloud app deploy --project html5rocks-hrd --no-promote ./app.yaml
This will deploy to a unique version and hostname but not take over serving yet. Check it works with your browser, then migrate all traffic to the new version from the Cloud Console.
⚠️ If you get a 500 when testing the site, wait ~10 seconds and try again. The code is slow to start, and App Engine can be initially unhappy.
If you need access to do this, ask in team chat or ping a lead.
If you're removing content and want to add a redirect, add it to the "_redirects.yaml" file. Changes to this file need the site to be redeployed as described above.