docs: add A2UI basic catalog implementation guide for v0.9

[jacobsimionato]

Description of Changes

• Introduced a new comprehensive guide (basic_catalog_implementation_guide.md) detailing how to visually and functionally implement the A2UI v0.9 Basic Catalog components and functions across Web, Mobile, and Desktop platforms.
• Specified rendering guidelines, property mapping, and behavior/state for all catalog components.
• Outlined the logic and reactive execution constraints for all catalog functions, including specific architecture guidance for the complex formatString interpolation engine.
• Established a "Leaf-Margin Strategy" to maintain consistent, predictable UI spacing without multiplication issues across nested layout containers.
• Added rules for handling color, contrast, and nesting, such as relying on native framework context for text contrast in buttons, and using transparent boundaries for nested cards.
• Updated the renderer_guide.md to link to this new document, specifically adding it to the required ingestion context and basic catalog implementation steps for AI Agents.

Rationale

Previously, the functional and visual expectations for implementing the Basic Catalog were vaguely scattered or entirely missing. Providing a detailed, framework-agnostic implementation guide ensures that developers and AI agents can build consistent, high-quality, cross-platform A2UI renderers. Specifically, codifying spacing rules and function reactivity reduces trial-and-error and ensures architectural alignment across different ecosystems.

Testing/Running Instructions

1. Review the new specification/v0_9/docs/basic_catalog_implementation_guide.md document for clarity and correctness.
2. Review the modifications in specification/v0_9/docs/renderer_guide.md to ensure the integration and links are correct.
3. This is a documentation-only change, so no automated code tests are required.