build: make doc target quiet

Currently it can be a little difficult to detect errors in the output
from the doc target. This commit suggests reducing the output to make it
easier to identify errors.

PR-URL: https://github.com/nodejs/node/pull/16516


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>