[Task]: Switch from PlantUML to Mermaid

[simonkurtz-MSFT]

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.
• We can only perform this work if there are Azure stencils that can be used with Mermaid.