CG-17133: Add Codegen Bot documentation

[codegen-sh]

This PR adds comprehensive documentation for the Codegen Bot in the docs/gen/ folder:

1. Added a detailed "How It Works" page explaining Codegen's architecture and workflow
2. Added a "Use Cases" page showcasing common ways to use Codegen
3. Updated mint.json to include the new documentation pages in the navigation

These changes will help users better understand how Codegen works and how to effectively use it in their development workflows.

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[codecov]

Codecov Report

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✅ All tests successful. No failed tests found.

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