Documentation improvement

Join all documentation (extensión .MD) in format https://readthedocs.org/

The issue names Markdown documentation and Read the Docs but no specific files or entry point. Start by inventorying the repository's .MD files and checking the existing documentation setup. Done means the documentation is joined and formatted for Read the Docs; confirm the expected structure before making changes.

索引モデルが issue の本文から書いたものです。