test: skip non-doc files in test-make-doc checks

PR-URL: https://github.com/nodejs/node/pull/21531
Fixes: https://github.com/nodejs/node/issues/21519


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>