doc: add watson to collaborators

Closes: https://github.com/nodejs/node/issues/18943

PR-URL: https://github.com/nodejs/node/pull/19234


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>