chore: redirect yari-demos.{prod,stage}.mdn.mozit.cloud -> developer.mozilla.org

[caugner]

Some links on these domains appeared on Google. The CNAME entries are already made in AWS Route 53.

Refractr PR Checklist

JIRA ticket: https://mozilla-hub.atlassian.net/browse/MP-1238

When creating a PR for Refractr, confirm you've done the following steps for smooth CI and CD experiences:

• Have you updated the relevant YAML in the PR?
• Have you checked the relevant YAML for any possible dupes regarding your domain?
• Have you checked if there are any TLS cert concerns - e.g. if the domain being redirected already exists, and it is being changed to point at Refractr, is a temporary TLS 'outage' while waiting for Lets Encrypt certification via HTTP challenge okay? If not, have you followed these steps for using DNS challenges with our cert-manager setup?
• If desired, have you generated the Nginx manually to confirm addition works as expected?
• If desired, are you able to connect to EKS (cluster itse-apps-prod-1, namespace fluxcd) to more closely monitor the deploys?

After PR merge, next steps include:

• If going straight from main merge & Stage deploy to a release & production deploy, create the relevant GitHub release with an incremented version / tag applied.
• Confirm you are ready and able to perform the requested DNS creation or change post-deploy?