Add llms.txt file to insight docs (#5887)

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title: "[SDK/Dashboard/Portal] Feature/Fix: Concise title for the changes"
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## Notes for the reviewer
Anything important to call out? Be sure to also clarify these in your comments.

## How to test
Unit tests, playground, etc.

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## PR-Codex overview
This PR focuses on enhancing the documentation and functionality related to `Agents & LLMs` within the `thirdweb Insight` API, including new links, improved examples, and standardized usage instructions for better integration with AI tools.

### Detailed summary
- Added a new link for `llms.txt` in `sidebar.tsx`.
- Updated documentation in `page.mdx` to clarify API usage for `Agents & LLMs`.
- Revised code examples to use template literals for dynamic values.
- Simplified `ChainId` type to a number.
- Enhanced error handling and response structure in API examples.
- Included detailed instructions on authentication and API endpoints.

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Author: AmineAfia