CCM-12397: Add sequence and flow diagram

Author: tdroza-nhs

docs/diagrams/digital-letters-flow.md

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+# Flow diagram
+
+```mermaid
+
+flowchart TD
+        Start(["Start"])
+        Start --> Mesh[Receive MESH file]
+        Mesh --> S3@{shape: disk, label: "Save to S3"}
+        S3 --> FileSize{"PDF < 8MB?"}
+        FileSize -->|Yes| PDM["Upload to PDM"]
+        FileSize -->|No| SupplierAPI@{shape: doc, label:"Send to SupplierAPI"}
+        PDM --> NHSApp["Send NHSApp msg"]
+        NHSApp --> PDFRead{"PDF Read?"}
+        PDFRead -->|Yes| End(["End"])
+        PDFRead -->|No| SupplierAPI
+        SupplierAPI --> End
+```

docs/diagrams/digital-letters-sequence.md

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+# Sequence diagram
+
+## Questions
+
+- 1. Should we validate the CommunicationRequest schema?
+- 2. (&14) This would result in two separate PDS hits
+- 3. Should Notify virus check the payload given that files >10MB don't go to PDM/NDR
+- 7. When does Notify delete its local copy?
+- 11. Duration and interval of polling
+- 21. Can NHSApp read callback go to notify-letters when the message request originated from notify-core?
+
+```mermaid
+
+sequenceDiagram
+  actor trust as Trust
+  participant notify-letters as Notify: Digital Letters
+  participant notify-core as Notify: Core
+  participant notify-supplier as Notify: Supplier API
+  participant pds as PDS
+  participant nhsapp as NHSApp
+  participant pdm as PDM
+  participant ndr as NDR
+
+  autonumber
+  trust -> notify-letters: MESH (CommunicationRequest)
+  notify-letters -> notify-letters: validate(schema, pdf)?
+  notify-letters -> notify-letters: virus-check?
+  opt Optionally check PDS
+    notify-letters ->> pds: GetPatient (NHSNumber)
+    pds -->> notify-letters: 200 OK (Patient)
+    notify-letters -> notify-letters: validate(death,sflag, rfr)
+  end
+  notify-letters ->> notify-letters: Store CommunicationRequest (S3)
+  notify-letters ->> pdm: POST /CommunicationRequest
+  activate pdm
+  pdm -) ndr: SFTP
+  pdm -->> notify-letters: 200 OK
+  deactivate pdm
+  Loop Interval & Duration TBC
+    notify-letters ->> pdm: GET /CommunicationRequest/<id>
+    pdm ->> notify-letters: 200 OK (CommunicationRequest)
+  end
+  rect rgb(204, 223, 241)
+    note over notify-letters,nhsapp: Existing Notify behaviour
+    notify-letters ->> notify-core: post /v1/messages (NHSApp)
+    activate notify-core
+      notify-core ->> pds: GetPatient(NHSNumber)
+      activate pds
+        pds -->> notify-core: 200 OK (Patient)
+      deactivate pds
+      notify-core -> notify-core: validate(death,sflag, rfr)
+      notify-core ->> nhsapp: sendMessage
+      nhsapp -->> notify-core: 201 CREATED
+      notify-core ->> notify-letters: 201 CREATED
+    deactivate notify-core
+  end
+
+  notify-letters ->> notify-letters: Create SendLetter TTL
+  alt Letter is read in NHS App
+    nhsapp ->> notify-letters: Callback(status: read)
+    notify-letters ->> notify-letters: Delete TTL
+  else Letter is not read after fallback wait time
+    notify-letters ->> notify-letters: SendLetter TTL expires
+    notify-letters ->> notify-supplier: SendLetter(PDF)
+  end
+```