[Docs cleanup] Refresh breaking-change / OAD guidance

**Context:** Breaking-change detection (OAD / `Azure/openapi-diff`) still applies to generated OpenAPI, but the
top-level guideline doc is already marked deprecated. This issue tracks pointing it at the current policy.

**Legend:** 🔴 OBSOLETE (delete/archive) · 🟡 NEEDS-UPDATE (reword)

**Tasks:**

- [ ] 🔴 `documentation/Breaking changes guidelines.md` — already marked "Deprecated"; replace with pointer to current policy
- [ ] 🟡 `documentation/BreakingChange-Oad-Rules-Mapping.md` — keep it for now (uses current `Azure/openapi-diff`); replace with typespec diff tool when it is ready.