Refactor Julia page (#1630)

* pull headings up one level to show julia and jupyter with equal space

* move native engine before jupyter engine

* move shared general info up

* add comparison table for both engines, be clearer about enabling native engine

* switch to plain markdown table

* `julia` and `jupyter` engine instead of native and Jupyter

* code style doesn't look good in table header

* capitalize Markdown

* default to julia

* make custom section a callout note to break the flow less

* explain server/worker relation better with flowchart

Author: jkrumbiegel