tex macro for MathJax3

[ianqsong]

I found tex macro for MathJax2 on #532. But my guess for MathJax3 failed as follows.

makedocs(
    ...
    format = Documenter.HTML(
        mathengine = MathJax3(Dict(
            :loader => Dict("load" => ["[tex]/physics"]),
            :tex => Dict(
                "tags" => "ams",
                "packages" => ["base", "ams", "autoload", "physics"],
                "macros" => Dict(
                    :ket => ["|#1\\rangle", 1], 
                ),
             ...

There are no warnings/errors to run make.jl but it just stop rendering tex code part. It wouldn't work using "Macros" either.

[ianqsong]

After 2-hour effort I found I should use other CDN for MathJax3 instead of https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.0.5/es5/tex-svg.js. So it might just be an version issue for MathJax. However on Documenter.jl side I suggest a little change #1428.