Improve new jO template and documentation
Changes introduced
- simplify/clarify new_jo template
- add missing CI job description to README
If more things need to be clarified let me know.
Tests
The new template can be seen here: https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/blob/doc/.gitlab/merge_request_templates/new_jO.md
Issues resolved
Closes #