Documentation: document modules

[sylvestre]

display, encoding, os, etc aren't documented here: https://uutils.github.io/dev/uucore/index.html

[Its-Just-Nans]

updated links

https://docs.rs/uucore/latest/uucore/

https://docs.rs/uucore/latest/uucore/display/index.html https://docs.rs/uucore/latest/uucore/io/index.html https://docs.rs/uucore/latest/uucore/os/index.html https://docs.rs/uucore/latest/uucore/shortcut_value_parser/index.html