Set up MyST documentation generation

[nokome]

Generate MyST markdown documentation from the JSON Schema definitions. Create documentation in docs/types/ with:

Generated files

• One markdown file per type (e.g., docs/types/Document.myst, docs/types/Heading.myst)
• Include type description, properties table, and examples
• Cross-link related types

Codegen integration

• Implement scripts/lib/generate-docs.ts
• Register in scripts/codegen.ts generators array
• Add pnpm codegen docs command

Documentation structure

• Title from schema title or type name
• Description from schema description
• Properties table with name, type, required, and description columns
• Examples section if schema has examples defined

Tasks

• Implement scripts/lib/generate-docs.ts
• Register docs generator in scripts/codegen.ts
• Create docs/types/ directory structure
• Run pnpm codegen docs and verify generated markdown
• Ensure cross-references between types work correctly

[rowanc1]

I took a first pass at this in #22, figured it was easier for me to do it!

• Has to be .md at the moment

I used schema instead of types as the folder, but only because I hadn't read this issue first..!