Jupyter-wide MathJax font choice

[isabela-pf]

In the Jupyter Notebook meeting today, we reviewed jupyter/notebook #5998 and found that

• one of the most compelling arguments for changing fonts is to create consistency across Jupyter projects
• there is not currently a consistent/singular/otherwise agreed upon approach for this across projects
• this PR is also linked to another PR asking to change fonts on JupyterLab

There seems to be a history of changing these fonts back and forth in some projects, so @afshin suggested that we agree on something and have a place to point people as to why the choice has been made to prevent changing this regularly in the future. Once a choice is made, I think it is worth adding a file to the repo to start tracking this and similar Jupyter-wide design choices.

I'm not very familiar with this problem and who at different projects might care about this, so I'd appreciate any help in getting those people involved.

[mgeier]

I think it would be great if MathJax version 3 could be supported in all Jupyter projects, which has a much better performance when a page with many equations is loaded.

Maybe this also makes the font problem obsolete?