Migrate architecture diagrams from PlantUML to Mermaid

[Copilot]

Replaces PlantUML with Mermaid for architecture diagrams, eliminating Java and Graphviz dependencies while enabling native Markdown rendering.

Changes

Diagrams

• Converted 4 architecture diagrams to Mermaid flowcharts
• Added Font Awesome icons (fa:fa-cloud, fa:fa-database, etc.) to compensate for lack of Azure stencils in Mermaid
• Preserved visual hierarchy via color coding: Azure blue (#0078D4) for services, light blue for apps, light green for APIs

Configuration

• Removed PlantUML settings from VS Code config (Java path, export paths, render mode)
• Added bierner.markdown-mermaid to recommended extensions

Documentation

• Documented that Mermaid lacks Azure-specific stencils (unlike PlantUML's Azure-PlantUML library)
• Explained Font Awesome icon support varies by renderer (full support in VS Code/GitHub, limited in Azure DevOps Wiki)
• Added guidance to use Draw.io/Visio with official Azure icons for high-fidelity branded diagrams

Example

flowchart LR
    apps([fa:fa-mobile Apps]):::appStyle
    apim[fa:fa-cloud API Management]:::azureStyle
    aca[fa:fa-box Container Apps]:::azureStyle
    
    apps -->|API Consumers| apim
    apim -->|Backend APIs| aca
    
    classDef appStyle fill:#ADD8E6,stroke:#333,stroke-width:2px
    classDef azureStyle fill:#0078D4,stroke:#333,stroke-width:2px,color:#fff

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Diagrams remain readable even when Font Awesome icons don't render, as service names are in text labels.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Task]: Switch from PlantUML to Mermaid</issue_title>
<issue_description>### Describe the task

• Switch from PlantUML to Mermaid for the architectural diagrams.
• Remove all PlantUML settings, files, etc.
• Add Mermaid extension for VS Code or create images from the Mermaid diagrams.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Task]: Switch from PlantUML to Mermaid #139

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