Add a Mermaid system diagram showing data flows and AWS resources

documentation/architecture/README.md

@@ -25,3 +25,44 @@ The current remote development environment uses [services provided by cloud.gov]
   reports, via [RDS PostgreSQL database](https://cloud.gov/docs/services/relational-database/).
 - **Task Queue Broker**: Manages queue of data validation tasks for Celery, via
   [AWS Elasticache Redis](https://cloud.gov/docs/services/aws-elasticache/).
+
+### System diagram
+
+```mermaid
+flowchart TB
+    %% NAD System Boundary
+    subgraph NAD_CH [NAD CH]
+        direction TB
+        %% Application and AWS Infrastructure
+        app[[Python/Flask Server]]
+
+        %% Storage Backends
+        postgres[(RDS Postgres)]
+        redis{{AWS Redis}}
+        s3[/S3 Bucket/]
+
+        %% Connections from Flask Application
+        app -.->|Stores provider and submission metadata| postgres
+        app -.->|Enqueues tasks| redis
+        app -.->|Reads and writes submissions| s3
+
+        %% Job Runner positioned below storage backends
+        job_runner[[Job Runner]]
+        job_runner -.->|Reads tasks| redis
+        job_runner -.->|Reads and writes data| s3
+    end
+
+    %% CI/CD System
+    cicd[(GitHub or GitLab)]
+    cicd -->|Deploys application stack - HTTPS| NAD_CH
+
+    %% External Components and Users
+    login[(Login.gov)]
+    data_provider{{Data Provider}}
+    nad_admin{{NAD Administrator}}
+
+    %% Interactions and Connections
+    data_provider -- Uploads Data Extracts - HTTPS --> app
+    nad_admin -- Manages Submissions & Providers - HTTPS --> app
+    app -->|Authenticates users - HTTPS| login
+```