mark clap-md as deprecated?

[jyn514]

https://github.com/clap-rs/clap-md hasn't been updated in 8 years and has several PRs that haven't been looked at in a while. glancing at #1979 it looks like clap doesn't intend to maintain this crate? would it make sense to mark it as deprecated so people don't try to use it?

[epage]

I've archived the repo.

The crate doesn't seem to be published, so not sure if there is much more to do.

[jyn514]

thanks!