build: add CODEOWNERS; edx/community-engineering

Background
==========
As part of our Squad-based ownership, we should stay on top of what
happens in these repositories. However, due to the number of
repositories (and subsequently pull requests) across the edX ecosystem,
it is challenging to stay on top of notifications, separating the
'signal' from the 'noise'. Email filters can go a long way to taming
Inbox notifications, but this is manual and requires maintenance as
Squad ownership changes. It also fails to account for Github-specific behavior.

Proposal
========
By leveraging Github support for `CODEOWNERS` files [1],
we can ensure that our team is at least CCed explicitly, here,
in the form a requested review. This request is just that, a request,
not a requirement; we are not enacting any new merge requirements
at this time.

- [1] https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners

.github/CODEOWNERS

+* @edx/community-engineering