Rd math rendering is broken in pkgdown 2.1.0

[bastistician]

Unfortunately, since pkgdown 2.1.0, Rd math like \eqn{f_r(r)} is only displayed as raw \(f_r(r)\) in the browser.
Compare, for example:

https://recipes.tidymodels.org/reference/step_harmonic.html#details (built with pkgdown 2.0.9)
https://recipes.tidymodels.org/dev/reference/step_harmonic.html#details (built with pkgdown 2.1.0)

This is related to the following major change in pkgdown 2.1.0:

• New template.math-rendering allows you to control how math is rendered across your site. The default uses mathml which is low-dependency, but has the lowest fidelity. You can also use mathjax, the previous default, and katex, a faster alternative. (Should it be easier to not load MathJax/to tweak the version? #1966).

A workaround is to include

template:
  math-rendering: mathjax

in the pkgdown config.

I guess the issue is that, e.g., pkgdown:::as_html.tag_eqn() simply wraps the equation in \( ... \), suitable for MathJax, but the new default engine, MathML, does not handle LaTeX syntax directly but requires special XML markup.

[bastistician]

Sorry for the noise: I just realized this is a duplicate of #2704.