Add comprehensive architecture documentation index for Planet platform

[wangkanai]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces a comprehensive architecture documentation index for the Planet geospatial platform, providing a structured overview of the system's architectural foundation, assessment, roadmap, and technical implementation guides. The documentation establishes a living index that consolidates architectural decisions, performance metrics, security posture, and strategic direction based on a detailed architectural assessment.

Key changes include:

• Complete architectural documentation framework with domain boundaries, health scores, and strategic roadmap
• Technical implementation guides covering DDD patterns, scalability strategies, and API documentation
• Quality standards, security architecture, and performance benchmarking sections
• Migration strategies for microservice extraction and decision records (ADRs)

Comments suppressed due to low confidence (2)

docs/ARCHITECTURE_INDEX.md:184

• [nitpick] The method name 'ValidateFormatAsync' is ambiguous. Consider renaming to 'ValidateImageFormatAsync' to be more specific about what format is being validated.

    Task<ValidationResult> ValidateFormatAsync(IRaster image);

docs/ARCHITECTURE_INDEX.md:208

• [nitpick] The parameter name 'NewTheme' uses Hungarian notation prefix. Consider renaming to 'Theme' for cleaner naming convention.

public record UserPreferencesChanged(int UserId, PlanetTheme NewTheme);