Added GCS sequence diagram

Author: apaillier-ledger

doc/gcs.md

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+# Generic Clear-Signing
+
+```mermaid
+sequenceDiagram
+    participant HW as Hardware Wallet
+    participant SW as Software Wallet
+
+    SW ->> HW: SIGN, store only
+    note over HW: store compressed<br/>calldata in RAM
+    HW ->> SW: OK / KO
+    SW ->> HW: TRANSACTION INFO
+    HW ->> SW: OK / KO
+    loop N times for N fields
+        SW ->> HW: token / NFT / enum / trusted name metadatas
+        HW ->> SW: OK / KO
+        SW ->> HW: TX FIELD DESCRIPTION
+        HW ->> SW: OK / KO
+    end
+    SW ->> HW: SIGN, start flow
+    note left of HW: check computed fields hash
+    note over HW: display all the<br/>formatted fields
+    HW ->> SW: OK (with r,s,v) / KO
+```