doc: revise Collaborator Nominations introduction

Revise the introductory paragraphs for Collaborator Nominations in
GOVERNANCE.md. This change splits long and unnecessarily complex
sentences into multiple simple statements.

PR-URL: https://github.com/nodejs/node/pull/27237


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>