docs: add references to punycode.md

Add references to related `url` functions in punycode.md. These provide
guidance to users who may be investigating the punycode module for URL
encoding.

PR-URL: https://github.com/nodejs/node/pull/36761


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Yash Ladha <yash@yashladha.in>
Reviewed-By: James M Snell <jasnell@gmail.com>