feat: mail service

mail/docs/README.md

@@ -0,0 +1,23 @@
+# Mail Service Docs
+
+This directory keeps service-local documentation that is more operational or
+more example-heavy than [`../README.md`](../README.md).
+
+Sections:
+
+- [Runtime and components](runtime.md)
+- [Main flows](flows.md)
+- [Configuration and contract examples](examples.md)
+- [Operator runbook](runbook.md)
+
+Primary references:
+
+- [`../README.md`](../README.md) for stable service scope, contracts, data
+  model, Redis layout, and retry policy
+- [`../api/internal-openapi.yaml`](../api/internal-openapi.yaml) for the
+  trusted internal REST contract
+- [`../api/delivery-commands-asyncapi.yaml`](../api/delivery-commands-asyncapi.yaml)
+  for the trusted async generic command contract
+- [`../../ARCHITECTURE.md`](../../ARCHITECTURE.md) for system-level service
+  boundaries and transport rules
+- [`../../TESTING.md`](../../TESTING.md) for the cross-service testing matrix