TeX markup rendering

[CarbonAlabel]

The Useful Formulae page should be updated to actually render the formulae.

One approach is to use a third-party service which renders them as images, such as @lukasni did in https://github.com/esi/esi-issues/blob/2c4462ed3aa68db8ab3c6d587756ee8eb4145f60/docs/useful_formulae.md.

Another is including a JS library which will render them, such as MathJax or KaTeX., You can find one of my attempts at this on https://carbonalabel.github.io/esi-docs/docs/useful_formulae.html and https://github.com/CarbonAlabel/esi-docs/blob/master/docs/useful_formulae.md.

If there's some third alternative, I'd love to hear about it.

The corresponding pages from the old docs can be used as a reference for what the rendered formulae are supposed to look like: https://eveonline-third-party-documentation.readthedocs.io/en/latest/formulas/

[lukasni]

...I never did finish that, did I?