Add Katex support to the build system, or consider alternatives

As discussed in #36, the build system currently lacks support for katex.

This means there's some markup on the [Merge Rules](https://automerge.org/docs/reference/under-the-hood/merge-rules/) page that isn't being rendered.

Naively adding katex is a bad idea — IIRC, it requires several fonts and masses of CSS, unwarranted for such a limited use.

One option would be to kick this can down the road — replace those handful of uses with carefully chosen unicode glyphs, or maybe some static images, or a tiny bit of custom HTML+CSS.

Another option would be to see if MathML (which is now baseline widely available) has the features we need.

Start by reading issue #36 and the Merge Rules page to identify the handful of unrendered KaTeX uses and the build-system entry point that processes them. Compare KaTeX, Unicode or static images, custom HTML/CSS, and MathML against the page's needs; done means those expressions render correctly without an unjustified dependency or styling burden.

Written by the indexing model from the issue text.