Clean up Scheduled Jobs docs

Created by: seancolsen

This page: https://docs.civicrm.org/sysadmin/en/latest/setup/jobs/

I brought stuff into this page from (a) the User Guide and (b) two different pages on the wiki. Now it's kind of a mess and has some duplicate content. Needs merging, cleaning, and some re-organizing.

Related: https://lab.civicrm.org/documentation/docs/issues/4