Archival of this repository

[pkra]

The rewrite for MathJax v3 (cf. #3 and #5) has become moot since the MathJax team plans to maintain the rewrite as part of MathJax-src (https://github.com/mathjax/MathJax-src/tree/mathtools_package).

Once that branch is part of a release, we should archive this repository.

[pkra]

With MathJax v3.2.0, this extension is now part of core MathJax https://github.com/mathjax/MathJax-src/releases/tag/3.2.0#packages.

[pkra]

We should add a note to the readme before archiving this repository.