feat: add thumbnail image

end-to-end-use-cases/long_context/book-character-mindmap/README.md

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 # Book Character Mindmap
 
-[![thumbnail](https://github.com/user-attachments/assets/11c6f1f3-59db-4638-9b1a-68f1d25efec4)](https://youtu.be/DL4-DswxfEM)
+![Book Character Mindmap](public/mindmap.png)
 
 Book Mind is a web application that allows users to explore character relationships and storylines in books using AI-powered visualizations.
 This leverages Llama 4's impressive 10M token context window to process entire books at once, enabling comprehensive analysis of complex narratives and character relationships across lengthy texts.