doc: use shorter fs.copyFile examples

This consolidates some examples to concentrate the reader on the
important aspects and to reduce reading overhead.

PR-URL: https://github.com/nodejs/node/pull/27044


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>