Use mathml as an alternative `mathengine` about documenter.jl HOT 3 OPEN

This seems like a reasonable option to have. If it works well, we can maybe even switch the default over.

Though maybe there are additional considerations to take into account (support for older browsers ?)

How old is old? I think that generally it's fine to assume the people run relatively recent browsers.

from documenter.jl.

From the compatibility standpoint, it seems that outside of chromium-based browser (support only dates back to the start of 2023 with version 109), there isn't an issue with compatibility for older browsers. (see https://www.igalia.com/2023/01/10/Igalia-Brings-MathML-Back-to-Chromium.html and https://developer.mozilla.org/en-US/docs/Web/MathML/Element/math)

For now, the display of math equation seems a little smaller using raw mathml compared to the other two engine, so some additional css might be needed to match the size.
I haven't checked how much time the conversion takes on math heavy sites (only that it works reasonably well)

from documenter.jl.

https://caniuse.com/mathml suggests 94% coverage. Perhaps people reading programming docs generally use up-to-date browsers though.

from documenter.jl.