Add Folder Markdown format support

[FBEZ]

Summary

Introduces a new "Folder Markdown" format that allows authoring multi-file course structures, enabling better team collaboration and organization for large courses. Includes auto-detection and example courses.

Key Changes

New Format: Folder Markdown

• Implements reader.py with a complete folder-based course compiler
• Reads hierarchical folder structure (course → chapters → sequentials → units)
• Merges multiple Markdown files into a single processed course
• Parses YAML frontmatter for metadata (title, order, hidden/draft flags)
• Automatically sorts content by order field

Index File Handling

• Course, chapter, and sequential index.md files are treated as metadata-only
• Body text in index files is ignored with warnings printed to console
• Only unit files (non-index markdown in sequentials) contain actual course content

Debug Mode

• Set MU_DEBUG_FOLDER_MD=1 environment variable to inspect merged markdown
• Keeps temporary files for debugging the merge process
• Prints file location to console for easy inspection

Documentation & Examples

• README section explaining folder structure and usage
• Updated installation instructions with virtual environment setup steps
• Added complete example course structure in course_folder
• Includes merged output example for reference

Project Maintenance

• Expanded .gitignore with comprehensive Python standards
• Added .DS_Store and muenv entries
• Added PyYAML dependency for YAML frontmatter parsing

Testing

Example courses can be tested with:

mu examples/course_folder examples_output/course_folder
MU_DEBUG_FOLDER_MD=1 mu examples/course_folder /path/to/olx/

Breaking Changes

None. The new format is additive and doesn't affect existing functionality.