Add MongoDB chat mode instructions

[madvimer]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, or chat mode file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, or chat mode with GitHub Copilot.
• I have run node update-readme.js and verified that README.md is up to date.

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Description

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Type of Contribution

• New instruction file.
• New prompt file.
• New chat mode file.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[aaronpowell]

Please ensure this is formatted as markdown appropriately, there are a lack of spaces between symbols. We use GitHub Flavoured Markdown.

Also, you need to run the update script to ensure the readme is updated.

[madvimer]

Please ensure this is formatted as markdown appropriately, there are a lack of spaces between symbols. We use GitHub Flavoured Markdown.

Also, you need to run the update script to ensure the readme is updated.

Hello - Formatted the changes and ran update script. Please review