🎨 Palette: Documentation accessibility scaffolding

[google-labs-jules]

💡 What: Improved the documentation template in .template/docs/template.md to use a more descriptive alt-text placeholder for architecture diagrams and added standard placeholders for Data Models and Environment Variables.
🎯 Why: In a template repository, the documentation scaffolding is the primary user interface for developers creating new functions. By providing an accessibility-first alt-text placeholder ('Architecture diagram for ${title_name} showing the data flow between components'), we promote a culture of accessibility by default.
♿ Accessibility: Ensures that all future templates generated from the scaffold start with a descriptive alt-text foundation for their architecture diagrams.

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PR created automatically by Jules for task 16940722769995989530 started by @amrabed

[google-labs-jules]

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