Use physics package for simpler equations

[sethaxen]

As suggested by @simeonschaub in #200 (comment), we could use the macros in the physics package, bundled with MathJax 3, to have simpler math expressions. If there are no objections, I'd like to do this.

[simeonschaub]

Ah, I completely forgot about that! If you want to give it a go, have a look at the Documenter tests I added here for how to set that up: https://github.com/JuliaDocs/Documenter.jl/blob/eea5e5c80084dbee66009d040b264d6e16a867ff/test/examples/make.jl#L275

[oxinabox]

Quoting in the relevant comment

https://github.com/JuliaDocs/Doumenter.jl/pull/1367 should be merged soon and my goal is to eventually use this with the physics package in ChainRules, so we don't always have to write \frac{\partial f}{\partial x} for every partial derivative. We could also make our differentials look a bit nicer – I personally am a big fan of non-italic d, but it's a bit of a pain to write in bare LaTeX. MathJax3 should also load almost as fast as KaTeX.