Move frontend-base ADR 0012 to OEP-10 #561

Description

ADR 0012 in frontend-base described a branching and publication strategy that applies to every semantically-released frontend repository in the Open edX org, not only to frontend-base itself, so it does not belong in a single repository's decision record. It moves to openedx-proposals as OEP-10 ADR 0002, leaving behind a stub that points at the published URL.

Part of #560. Proposal: openedx/openedx-proposals#815

Implemented by openedx/frontend-base#282