Add support for `markdown_to_blockdoc` conversion

• New markdown_to_blockdoc converter for transforming Markdown documents to BlockDoc format
• Comprehensive parser that handles all core Markdown elements:
  • Headings (ATX and Setext styles)
  • Paragraphs with proper line break handling
  • Lists (ordered and unordered)
  • Code blocks with language detection
  • Block quotes with attribution
  • Images with captions
  • Horizontal rules
• Semantic ID generation based on content
• Example implementation in examples/markdown_conversion_example.py
• Test suite for the converter functionality