docs: Add DaoCloud mirror proxy instructions to README_CN.md

[kenwoodjw]

Summary

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• Added information about using the DaoCloud mirror proxy (docker.m.daocloud.io) in the "Quick Start" section of README_CN.md
• Included a link to the DaoCloud public image mirror repository for reference
• Provided clear instructions on how to modify the docker-compose.yaml file to use the mirror

Close https://github.com/langgenius/dify/issues/14615

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Checklist

[!IMPORTANT]
Please review the checklist below before submitting your pull request.

• [ ] This change requires a documentation update, included: Dify Document
• [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods