Adding MathJax3.

Current port www/mathjax is for now legacy version, v2. Current version
of MathJax is v3. v2 is still supported and gets patch releases, but v3
is intended for new deployments.

Upgrading v2 to v3 is not in-place upgrade, url of the script changes,
which also requires change config of web server which hosts it.
Therefore I think it is better to introduce new version as mathjax3.

PR: 248796
Submitted by: Marko Cupać (maintainer)