JEP template improvements

[Basil Crow]

Not a JEP in and of itself, but some improvements to the JEP template:

- https://github.com/jenkinsci/jep/pull/401 Improve section order and
add clarity to existing sections
- https://github.com/jenkinsci/jep/pull/402 Add work estimate section
to template
- https://github.com/jenkinsci/jep/pull/403 Add migration section to template

I also intend, as time permits, to create a fourth PR to break up the
monolithic "Reasoning" section into distinct "Evaluation" and
"Solution" sections, as well as to retroactively file a JEP for the
completed Prototype removal project.