docs: create vault backend ADR

docs/ADR/002-Vault-Backend.md

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+# 002-Vault-Backend
+
+## Status
+
+
+## Context
+
+Currently vault Backend is onboard in Consul KV
+Hashicorp recommandation is to use integrated storage from vault cluster
+This could remove consul dependancy on rebuild
+
+## Decision
+
+migrate to vault integrated storage
+
+## Consequences
+
+to do:
+- migration plan https://developer.hashicorp.com/vault/tutorials/raft/raft-migration
+- backup https://developer.hashicorp.com/vault/tutorials/standard-procedures/sop-backup
+