[web] Formula fixes

• Updated to MathJax 3
• Fixed some formula rendering issues (Sorry @cbsilver that I missed some of your fixes, but now they should be included)
• Preview is here

Also please note (thanks to @cbsilver for pointing this out) that you either can start a formula block with $$ or with \begin{equation}.

A few issues remain, and I would like to ask the corresponding authors to take a look:

• benchmarks/thermo-hydro-mechanics/consolidation_pointheatsource.md @joergbuchwald
• benchmarks/heatconduction/bhe_array_benchmark.md @HBShaoUFZ
• benchmarks/hydro-component/contracer/contracer.md @joboog
• benchmarks/reactive-transport/wetland/wetland.md @joboog
• benchmarks/reactive-transport/kineticreactant_allascomponents/kineticreactant2.md @joboog
• benchmarks/creepbgra/creepbrga.md @wenqing

See https://www.opengeosys.org/docs/devguide/documentation/introduction/ on how to test this locally.

1. Feature description was added to the changelog
2. Any new feature or behavior change was documented?

Follow-up on !3267 (closed).