Add MathJax3 Writer

[simeonschaub]

It would be good to support MathJax 3 for LaTeX rendering as well. My main motivation behind this is that MathJax 3 officially supports the physics package as a plugin. From quick testing, it also seems to render a lot faster than MathJax 2 and really not much slower than KaTeX. It's probably better to add alongside MathJax 2, since the config formats are not compatible, so this would otherwise be breaking. I'd like to take a stab at this, if that's ok.

[mortenpi]

Yes, this would be good. Going forward, MathJax 3 should probably be the default MathJax version anyway. But we should give people the option to also keep using MathJax 2.

From the API perspective we have two options I guess:

• Make it possible to pass a version number to MathJax and then check for that in mathengine!.
• Create a new type, e.g. MathJax3. In this case, we could also deprecate MathJax in favor of MathJax2.

I think either should be a pretty straightforward addition to here:

Documenter.jl/src/Writers/HTMLWriter.jl

Lines 369 to 468 in f9f0b68

	"Provides a namespace for remote dependencies."
	module RD
	using JSON
	using ....Utilities.JSDependencies: RemoteLibrary, Snippet, RequireJS, jsescape, json_jsescape
	using ..HTMLWriter: KaTeX, MathJax
	const requirejs_cdn = "https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.6/require.min.js"
	const google_fonts = "https://fonts.googleapis.com/css?family=Lato|Roboto+Mono"
	const fontawesome_version = "5.11.2"
	const fontawesome_css = [
	"https://cdnjs.cloudflare.com/ajax/libs/font-awesome/$(fontawesome_version)/css/fontawesome.min.css",
	"https://cdnjs.cloudflare.com/ajax/libs/font-awesome/$(fontawesome_version)/css/solid.min.css",
	"https://cdnjs.cloudflare.com/ajax/libs/font-awesome/$(fontawesome_version)/css/brands.min.css",
	]
	const jquery = RemoteLibrary("jquery", "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/jquery.min.js")
	const jqueryui = RemoteLibrary("jqueryui", "https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.12.1/jquery-ui.min.js")
	const lunr = RemoteLibrary("lunr", "https://cdnjs.cloudflare.com/ajax/libs/lunr.js/2.3.6/lunr.min.js")
	const lodash = RemoteLibrary("lodash", "https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.min.js")
	# headroom
	const headroom_version = "0.10.3"
	const headroom = RemoteLibrary("headroom", "https://cdnjs.cloudflare.com/ajax/libs/headroom/$(headroom_version)/headroom.min.js")
	const headroom_jquery = RemoteLibrary(
	"headroom-jquery",
	"https://cdnjs.cloudflare.com/ajax/libs/headroom/$(headroom_version)/jQuery.headroom.min.js",
	deps = ["jquery", "headroom"],
	)
	# highlight.js
	"Add the highlight.js dependencies and snippet to a [`RequireJS`](@ref) declaration."
	function highlightjs!(r::RequireJS, languages = String[])
	# NOTE: the CSS themes for hightlightjs are compiled into the Documenter CSS
	# When updating this dependency, it is also necessary to update the the CSS
	# files the CSS files in assets/html/scss/highlightjs
	hljs_version = "9.15.10"
	push!(r, RemoteLibrary(
	"highlight",
	"https://cdnjs.cloudflare.com/ajax/libs/highlight.js/$(hljs_version)/highlight.min.js"
	))
	prepend!(languages, ["julia", "julia-repl"])
	for language in languages
	language = jsescape(language)
	push!(r, RemoteLibrary(
	"highlight-$(language)",
	"https://cdnjs.cloudflare.com/ajax/libs/highlight.js/$(hljs_version)/languages/$(language).min.js",
	deps = ["highlight"]
	))
	end
	push!(r, Snippet(
	vcat(["jquery", "highlight"], ["highlight-$(jsescape(language))" for language in languages]),
	["\$", "hljs"],
	raw"""
	$(document).ready(function() {
	hljs.initHighlighting();
	})
	"""
	))
	end
	# MathJax & KaTeX
	const katex_version = "0.11.1"
	const katex_css = "https://cdnjs.cloudflare.com/ajax/libs/KaTeX/$(katex_version)/katex.min.css"
	function mathengine!(r::RequireJS, engine::KaTeX)
	push!(r, RemoteLibrary(
	"katex",
	"https://cdnjs.cloudflare.com/ajax/libs/KaTeX/$(katex_version)/katex.min.js"
	))
	push!(r, RemoteLibrary(
	"katex-auto-render",
	"https://cdnjs.cloudflare.com/ajax/libs/KaTeX/$(katex_version)/contrib/auto-render.min.js",
	deps = ["katex"],
	))
	push!(r, Snippet(
	["jquery", "katex", "katex-auto-render"],
	["\$", "katex", "renderMathInElement"],
	"""
	\$(document).ready(function() {
	renderMathInElement(
	document.body,
	$(json_jsescape(engine.config, 2))
	);
	})
	"""
	))
	end
	function mathengine!(r::RequireJS, engine::MathJax)
	push!(r, RemoteLibrary(
	"mathjax",
	"https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.6/MathJax.js?config=TeX-AMS_HTML",
	exports = "MathJax"
	))
	push!(r, Snippet(["mathjax"], ["MathJax"],
	"""
	MathJax.Hub.Config($(json_jsescape(engine.config, 2)));
	"""
	))
	end
	mathengine(::RequireJS, ::Nothing) = nothing
	end

[simeonschaub]

I think since MathJax3 uses a completely different config format, I though option 2 made more sense. I would love to hear your initial feedback on my draft.