[Docs cleanup] Refresh breaking-change / OAD guidance

[lirenhe]

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.