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.

Description

This pull request adds a highly detailed, practical, and engineer-authored API Design & Documentation Generator prompt.
It covers REST and GraphQL best practices, OpenAPI/Swagger generation, documentation standards, testing examples, client SDK generation, and more.
The README has been updated to include this new resource.

• Follows all contribution, security, and support guidelines.
• README and YAML front matter are up to date.
• No unwanted files included.

Type of Contribution

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

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