Manpage generation

[tertsdiepraam]

Like clap we should be able to create manpages automatically.

[tertsdiepraam]

I'll expand a bit on this on why this is difficult in general. We can have the proc macro for Arguments do this automatically, but the information is non-local. For example, the accepted values come from the Value macro and are not encoded within the arguments enum. So we need to combine the output of multiple macros somehow. Here's my proposed solution:

1. Each macro writes JSON to the target directory, so that we get a JSON file per type.
2. We provide an additional library (and maybe even binary) that collects these files and builds manpages and shell completions out of that.
3. In uutils, we use that library in uudoc.

This crate will be useful: https://github.com/rust-cli/man

Edit: Well, I just realized I wrote the same plan as in https://github.com/tertsdiepraam/uutils-args/issues/4. This solution will probably do both completions and manpages at the same time.