docs: add pnfs flow diagram

modules/dcache-nfs/README.md

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+# NFSv4.1/pNFS Door
+
+
+```mermaid
+sequenceDiagram
+    autonumber
+    actor Client
+    participant Door
+    participant poolA
+    participant PoolManager
+    Client->>Door: OPEN(file), LAYOUTGET 
+    Door->>PoolManager: Select Pool
+    PoolManager-->>Door: 'poolA'
+    Door->>poolA: Start Mover (state)
+    poolA-->>Door: Ready(id)
+    Door-->>Client: 'open state', 'layout'
+    loop Application IO
+        Client->>poolA: READ(file, state, offset, len)
+        poolA-->>Client: 'bytes'
+    end
+    Client->>Door: LAYOUT_RETURN, CLOSE
+    Door->>poolA: Kill Mover(id)
+    poolA->>poolA: state invalidated
+    poolA-->>Door: Trnaster Finish
+    Door->>Door: Invalidate state
+    Door-->>Client: OK
+```