🎨 Palette: Improve documentation accessibility and technical clarity

[google-labs-jules]

💡 What:

• Added descriptive text to HTTP response codes in the REST API documentation (e.g., 200 (OK)).
• Added a text-based link for the "Use this template" action in the main README to support screen readers.
• Standardized data model tables across all templates to include technical character length constraints (e.g., 1-50 chars).
• Added a missing Item model table to the GraphQL documentation for consistency.

🎯 Why:

• Documentation is the primary UI for this template repository. These small touches make it more intuitive, accessible, and informative for developers.
• Explicit character limits help developers understand data validation requirements without digging into the source code.
• Descriptive response codes and text links improve the experience for screen reader users and provide better scannability.

♿ Accessibility:

• Added text fallback link for the "Use this template" image button.
• Improved accessibility of the Endpoints table by providing human-readable status code descriptions.

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PR created automatically by Jules for task 10825449826129387297 started by @amrabed

[google-labs-jules]

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