use hugo instead, gitbook is too slow to build

Long term schedule

The issue names no files or tests and only proposes using Hugo instead of GitBook because builds are slow. Start by inspecting the repository's current GitBook configuration and documentation build entry points. Done means the documentation builds with Hugo and the long-term migration scope is complete, but the issue does not define that scope.

索引モデルが issue の本文から書いたものです。