Generate Volta man pages

[chriskrycho]

We do not currently supply man pages for Volta. However, it would be relatively straightforward to implement using the clap_mangen crate, since we already use clap for our CLI parser. If someone wants to implement it, it should be a relatively easy first issue.

For a rough idea of how to implement it, you would want to do roughly what we do in src/command/completions.rs:

• Add clap_mangen as a dependency for that crate.
• Add a new man_pages module as a sibling to completions and the others.
• Define a new clap argument with a pub(crate) struct ManPages.
• Implement Volta’s Command trait for ManPages, including logging events the same basic way Completionts does (so you will also need to add a new ActivityKind etc.) for it.
• Use the Man struct from clap_mangen to generate the man pages from the command (crate::cli::Volta::command()).
• Emit the generated roff to the appropriate location. (You will have to see where that is on a per-OS basis!) One note that adds a small bit of complexity here: there is no “default” place to install it on Windows, as far as I know, but we should presumably still support it for someone who wants to read them there.

(Originally posted as a comment by @chriskrycho in #1708 (comment))