feat: modularize system-versioned-tables, mcp and sdd artifacts

[looreenz]

Final phase of the modular architecture implementation and repository cleanup.

Key Changes:

• Full Modularization: Completed the migration for mariadb-system-versioned-tables and mariadb-mcp using the established docs/ pattern.
• Documentation Artifacts: Included SDD documentation (sdd/) to provide transparency on the transformation process for future maintainers.
• Verification: Final pass ensuring all 8 root SKILL.md files remain under the 100-line threshold while maintaining valid relative links to all modular documentation.
• Outcome: The repository now features a scalable documentation architecture where each component is decoupled, easy to maintain, and optimized for AI agent context ingestion.