mangen command

[ken-matsui]

It would be great if we could generate man files (ROFF) using and modifying CLI classes, similar to clap_mangen. This command is intended for use by package managers, such as Homebrew, during the installation of Poac, not for typical end users. Therefore, it would be preferable to hide this command by default, like we do for some options.

https://github.com/poac-dev/poac/blob/7c4f62ef0902c763571cff39576c46c2bccb8827/src/main.cc#L22-L25

https://github.com/poac-dev/poac/blob/7c4f62ef0902c763571cff39576c46c2bccb8827/src/Cli.hpp#L61-L64