feat: add DocumentRenderer with math and code highlighting support
- Add katex and shiki dependencies for LaTeX math and syntax highlighting
- Export DocumentRenderer component from library index
- Replace custom content rendering with DocumentRenderer in document page
- Apply code formatting fixes across documentation and examples
This introduces proper rich text rendering capabilities including:
- Math equations via KaTeX
- Syntax highlighting via Shiki
- Headers, lists, blockquotes with theme support
- Links, mentions, and inline formatting
Breaking changes: None - this is additive functionality