[WIP] mdx -- consider adding output to pdf or docx or pptx abilities

[copilot-developer-agent-robch]

Plan to implement markdown export functionality:

• Create new IMarkdownExporter interface to define export contract
• Create concrete exporters for each format:
  • PdfMarkdownExporter to handle PDF output
  • DocxMarkdownExporter to handle DOCX output
  • PptxMarkdownExporter to handle PPTX output
• Add markdown parsing functionality using Markdig library
• Add export command to program options
• Update help documentation with new export capabilities
• Add unit tests for exporters

Fixes #6.