build: lint all docs under doc

`**` expansion doesn't behave as expected and as a result files in
nested subdirectories under `doc` were not linted. Use `find` instead
to generate the list of files to lint.

PR-URL: https://github.com/nodejs/node/pull/28128
Refs: https://github.com/nodejs/node/pull/28127


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>