Update docs changelog (for PR #57008) (#57092)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: hubwriter <[email protected]>
Co-authored-by: Sunbrye Ly <[email protected]>

Author: 4 people

CHANGELOG.md

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 **12 August 2025**
 
+OpenAI GPT-5 is now available in public preview for GitHub Copilot. GPT-5 is slowly rolling out to all paid Copilot plans and you will be able to access the model in GitHub Copilot Chat on github.com and Visual Studio Code (Agent, Ask, and Edit modes). 
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+See [Supported AI models in Copilot](https://docs.github.com/en/copilot/reference/ai-models/supported-models).
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+<hr>
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+**12 August 2025**
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 We’ve updated the documentation for Copilot repository custom instructions to go with the release that now brings this feature to the Eclipse IDE.
 
 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).