This commit adds a new build configuration/deployment mode (`rc`) for deploying release-candidate versions to angular.io. This is part of the work needed to prepare angular.io for our [new versioning/branching process][1] (also tracked in #39366). [1]: https://docs.google.com/document/d/197kVillDwx-RZtSVOBtPb4BBIAw0E9RT3q3v6DZkykU PR Close #39470