docs(select): add List MID Insights API documentation page

[bertolo1988]

Summary

Adds a new documentation page for the List MID Insights API endpoint.

This page covers:

• Endpoint specification and authorization
• All request parameters (headers, path, query)
• Full response object field reference
• MID origin types table
• Scheme-specific response fields
• curl implementation example
• Standard error codes

Jira: FT-1413

Type of change

• New feature / documentation

Checklist

• Content is accurate and based on approved PDF specification
• Styled consistently with existing docs pages

[Copilot]

Pull request overview

Adds a new Select API documentation page for the List MID Insights endpoint, describing eligibility, request parameters, response fields, origin types, and example usage.

Changes:

• Introduces select/mid-insights.md with endpoint overview, requirements, and request parameter tables
• Documents response object fields plus scheme-specific identifiers
• Adds MID origin type reference, curl example, and standard error codes

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