docs: Create docs/README.md

[KemingHe]

Problem

Design documents directory needs self-contained entry point explaining purpose and structure of living design documents.

Proposed Solution

Create docs/README.md following readme-creation skill:

• Purpose: Explain what living design documents are and why they exist
• Structure: Brief description of three document types (Core Decisions, Taxonomy, Parked)
• Usage: When to read each document, how they relate
• Audience: Internal planning artifacts for maintainers and contributors
• Links: Direct links to each design-*.md file

Target: Under 50 lines (directory README, not root).

Self-containment: Should make sense without reading parent README.

Alternatives Considered

Documenting design doc purpose in root README, but docs/ should be self-explanatory as standalone directory.