🎨 Palette: Add Back-to-Overview navigation to template documentation

[google-labs-jules]

💡 What: Added persistent "← Back to Overview" navigation links to the bottom of all template documentation pages (docs/template/*.md) and the base documentation template (.template/docs/template.md).

🎯 Why: To make navigating back to the main documentation index (docs/README.md) smooth, intuitive, and highly accessible without requiring users to use the browser back button or manually edit the URL.

♿ Accessibility: Ensures that screen readers and keyboard-only users can easily return to the overview page from any deeply-nested template page via standard tab-navigable links.

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PR created automatically by Jules for task 6927609240662466243 started by @amrabed

[google-labs-jules]

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