feat: Add demo infrastructure and documentation

[jeremyeder]

Summary

• Add comprehensive demo and documentation infrastructure
• Add demo documentation files (eval harness, quickref, summary)
• Add build automation with Makefile
• Add documentation demos (slides, walkthrough, terminal demo)
• Add demo generation scripts (build, slides, recording)
• Add Mermaid diagram support for documentation
• Add test coverage for demo generation
• Update markdown link checker to ignore Jekyll internal links

Test Plan

• All pre-commit hooks pass (trailing whitespace, black, isort, ruff)
• Markdown link checker passes with updated ignore patterns
• Conventional commit format verified
• Demo scripts can be executed successfully
• Documentation site builds correctly with new demos
• Test suite passes (test_demo_generation.py)

🤖 Generated with Claude Code