Re-write content on scheduled jobs

Created by: seancolsen

I'm trying to do some major improvements on the docs for scheduled jobs, but this content is very tricky to organize! I've put a lot of hours into this already, but I think I need some help from others because I'm unsure about a few things here. Tagging @MegaphoneJon @mlutfy @joannechester @michaelmcandrew @seamuslee001 to see if any of you want to help me answer some of the outstanding questions here.

Related PR

• This User Guide PR references sections in the jobs.md Sysadmin Guide page in this PR.

Source material

This PR is mostly taking content from the following sources and re-organizing it into two pages that clearly target system administrators:

• https://wiki.civicrm.org/confluence/display/CRMDOC//Managing+Scheduled+Jobs
• https://wiki.civicrm.org/confluence/display/CRMDOC//Running+Command-line+Scripts+via+URL
• https://github.com/civicrm/civicrm-user-guide/blob/3e12f820be20540162ef3f99e05678279d91a422/docs/initial-set-up/scheduled-jobs.md

Questions

1. Overall, I'd love to have someone look over this. It's been a few years since I set up scheduled jobs on a site.
2. I have a bunch of "TODO" comments in docs/setup/jobs.md. I could use help with each of these.
3. Should we even bother to document cv, as I started to do here?
4. Why would people use cron.php instead of using the REST interface to call Job.execute? Because if the answer is "cron.php only exists for legacy reasons" then I wonder whether we should instead be directing readers to use the REST interface and only mention cron.php briefly.

This PR should resolve #20 (closed)