doc: improve guide text for CI runs

Make the text about CI runs in the COLLABORATOR_GUIDE more concise. Add
information about Resume Build which should generally be preferred over
Rebuild.

PR-URL: https://github.com/nodejs/node/pull/21645


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