[DOCS] Update API reference documentation for multi-arch builds

[adambkaplan]

Overview

This task updates the API reference documentation to include new fields and changes for multi-arch builds, as described in SHIP-0043.

Description

Update API reference documentation to:

• Document new architectures field in Build and BuildRun specs
• Explain architecture field behavior and defaults
• Document any API version changes
• Update OpenAPI/Swagger specifications if applicable

Acceptance Criteria

• API reference updated with new fields
• Field descriptions and examples included
• API version documentation updated
• OpenAPI/Swagger specs updated if applicable
• Documentation reviewed and approved

Dependencies

• Blocked by: Implement multi-arch build API, orchestration, and node selection

Technical Notes

• Follow existing API reference documentation format
• Include field validation rules and constraints
• Provide clear examples for each field
• Ensure documentation is generated from code where possible

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Generated-by: Cursor

[adambkaplan]

Related to shipwright-io/community#285 - SHIP-0043 Multi-arch Image Builds