Add Brands and Email Settings API documentation to Unstable version

[VedranZoricic]

Context

• https://github.com/intercom/intercom/issues/421822

Summary

• Added comprehensive OpenAPI documentation for new Brands and Email Settings APIs
• These endpoints are added to the Unstable version only as they are new features
• Documentation follows existing patterns and conventions from the codebase

Changes

New API Endpoints (4 total)

• GET /brands - List all brands in the workspace
• GET /brands/{id} - Retrieve a specific brand by ID
• GET /emails - List all email settings for the workspace
• GET /emails/{id} - Retrieve a specific email setting by ID

Schema Definitions

• brand - Individual brand object schema
• brand_list - List container for brands
• email_setting - Individual email setting object schema
• email_list - List container for email settings

API Tags

• Added Brands tag for brand-related operations
• Added Emails tag for email settings operations

Technical Details

• All endpoints include proper authentication requirements
• Comprehensive examples provided for each endpoint
• Error responses follow standard Intercom error format
• Unix timestamps used for date fields (consistent with existing APIs)
• String IDs used throughout (consistent with existing patterns)
• Nullable fields properly marked in schemas

Testing

• OpenAPI specification validated with @apidevtools/swagger-cli
• All schema references resolve correctly
• Endpoints placed at end of paths section for easier review

Related Documentation

This PR implements the API documentation based on the specifications provided in the internal documentation files.

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