Add CODEOWNERS file to protect secrets

We need to make sure only staff can access secrets and/or deploy to
production.

This commit adds a [CODEOWNERS file] to ensure that any changes to our
GitHub Actions are reviewed by a developer, to avoid the (unlikely)
scenario where a PR that changes our GitHub Actions is approved by
someone who isn't a developer and doesn't spot the significance of the
change.

[CODEOWNERS file]: https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-code-owners

CODEOWNERS

@@ -0,0 +1,2 @@
+# Workflows (which have access to secrets and to production)
+.github/workflows/  @alphagov/design-system-developers