docs: add dcache door generic workflow

modules/dcache/README.md

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+# dCache request diagrams
+
+
+## Door workflow on successful read, close through door (NFS)
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant Client
+    participant Door
+    participant poolA
+    participant PoolManager
+    participant PnfsManager
+    participant Billing
+    Client ->> Door: GET
+    Door->>PnfsManager: PnfsGetFileAttributes (path|pnfsid)
+    PnfsManager -->> Door: storage info, attrs
+    
+    Door ->> PoolManager: PoolMgrSelectReadPoolMsg(pnfsid, storage info)
+    PoolManager-->>Door: poolA
+    Door->>poolA: Start Mover (pnfsid, protocol, storage info)
+    poolA -->>Door: mover id
+    poolA-->>Door: redirect
+    Door -->> Client: redirect
+    loop Application IO
+        Client->>poolA: GET
+        poolA-->>Client: 'bytes'
+    end
+    Client->>Door: CLOSE
+    Door->>poolA: Kill Mover(id)
+    poolA ->> Billing: MoverInfoMessage
+    poolA-->>Door: DoorTransferFinishedMessage(status)
+    Door->>Billing: DoorRequestInfoMessage
+    Door-->>Client: OK
+```
+
+## Door workflow on successful read, close through pool (dcap, xroot, http)
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant Client
+    participant Door
+    participant poolA
+    participant PoolManager
+    participant PnfsManager
+    participant Billing
+    Client ->> Door: GET
+    Door->>PnfsManager: PnfsGetFileAttributes (path|pnfsid)
+    PnfsManager -->> Door: storage info, attrs
+    
+    Door ->> PoolManager: PoolMgrSelectReadPoolMsg(pnfsid, storage info)
+    PoolManager-->>Door: poolA
+    Door->>poolA: Start Mover (pnfsid, protocol, storage info)
+    poolA -->>Door: mover id
+    poolA-->>Door: redirect
+    Door -->> Client: redirect
+    loop Application IO
+        Client->>poolA: GET
+        poolA-->>Client: 'bytes'
+    end
+    Client->>poolA: CLOSE
+    poolA ->> Billing: MoverInfoMessage
+    poolA-->>Door: DoorTransferFinishedMessage(status)
+    Door->>Billing: DoorRequestInfoMessage
+    opt dcap
+        Door-->>Client: OK
+    end
+```