docs: Add deprecation notice for repo.

[dianakhuang]

Also removes this repository from being
tagged for Open edx releases.

Configuration Pull Request

Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.
• Think about how this change will affect Open edX operators and update the wiki page for the next Open edX release if needed

[kdmccormick]

We can't remove a repo from the release unless it completes the DEPR process. Let's work on getting openedx/public-engineering#51 to completion instead--assuming it does, we can just archive it to openedx-unsupported, and then you folks can fork it into the edx org.