docs: add a mermaid plot to the README documenting the API architecture

[jdhoffa]

See an example here:

https://github.com/RMI/engageAct-API/blob/ac04759b3dab5115a4ba49361ac8cf43f7ead0c1/README.md?plain=1#L108-L122

[jdhoffa]

For example:

graph TD
    subgraph Local Development Env.
        A[Developer's Laptop]
        B[Docker]
        C[FastAPI Application]
        A --> B
        B --> C
    end

    subgraph Azure Cloud
        subgraph App Services
            D[Azure App Service]
            E[FastAPI Application Container]
            D --> E
        end

        subgraph Backing Services
            F[Azure Blob Storage]
            G[Azure Application Insights]
            H[Azure Key Vault]
        end
    end

    subgraph GitHub
        I[CI/CD by GH Actions]
        J[Azure Container Registry]
        I -->|GH Action Push to ACR| J
        J --> D
    end

    C -->|GH Push| I
    E -->|Fetch Data| F
    E -->|Logging & Monitoring| G
    E -->|Secret Management| H

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