Update CHANGELOG with new tutorial for Mermaid diagrams

Added a tutorial for using Copilot to create Mermaid diagrams.

Author: hubwriter

CHANGELOG.md

@@ -6,6 +6,10 @@ We’ve updated the documentation for Copilot repository custom instructions to
 
 See: [Adding repository custom instructions for GitHub Copilot](https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions?tool=eclipse) and [About customizing GitHub Copilot Chat responses](https://docs.github.com/en/copilot/concepts/response-customization?tool=eclipse).
 
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+**12 August 2025**
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 We have added a tutorial for using Copilot to create Mermaid diagrams at [Creating Diagrams](https://docs.github.com/en/copilot/tutorials/copilot-chat-cookbook/communicate-effectively/creating-diagrams).
 
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