[FR] New way to customize LaTeX preamble

[inkydragon]

People sometimes need to modify existing preamble sections to achieve some complex effects:

• Adjusting the TOC style: inkydragon/julia at doc-pdf-toc+compact-style
• Add a cover page: en-cover-page
• Add Chinese language support: julia-doc-zh-cn.tex

I customize the preamble by overriding the writeheader function.

Maybe Documenter could accept a keyword argument specifying the path to a custom preamble.tex and use its contents to replace the existing preamble.

Originally posted by @inkydragon in JuliaLang/julia#43698 (comment)

[mortenpi]

I think it would be completely fine to add such an option to the LaTeX struct:

Documenter.jl/src/Writers/LaTeXWriter.jl

Lines 43 to 49 in bffc866

	struct LaTeX <: Documenter.Writer
	platform::String
	function LaTeX(; platform = "native")
	platform ∈ ("native", "docker", "none") || throw(ArgumentError("unknown platform: $platform"))
	return new(platform)
	end
	end

[inkydragon]

Do we need to add a new platform?

Maybe we just need to add a new optional input file, like custom.sty.