Use tectonic to build the pdf version of the manual

[ViralBShah]

Given that tectonic is now in Yggdrasil, it might be nice to move towards using it for building the pdf version of the manual.

Some discussion on how to do that is in JuliaDocs/Documenter.jl#1785 (comment)

[fredrikekre]

Probably requires changes in Documenter? Perhaps move there or at least have an issue there too.

[inkydragon]

xref: Migrate the PDF docs build to Buildkite JuliaCI/julia-buildkite#198

[nsajko]

I'd caution against doing this.

Firstly - "use tectonic to build the PDF" is a misleading way of saying "move from LaTeX to a new and experimental TeX variant". While Tectonic is marketed as a more modern way of building (La)TeX documents, it's actually more than that, it's yet another implementation of TeX, but one somewhat divorced from the common conventions underpinning the TeX ecosystems.

An analogy: adopting Tectonic for building the PDF docs would be like adopting a peculiar and nonstandard C compiler for building the parts of Julia implemented in C/C++. While this seems harsh, it is perhaps even charitable to the option of moving to Tectonic.

From a TeX engine perspective, Tectonic is a fork of the long abandoned (but still used) TeX engine called XeTeX. The thing is, XeTeX wasn't abandoned just by its maintainers, it's also largely abandoned by the ecosystem of LaTeX packages and other TeX-related software. While XeTeX still "works" for some use cases to some degree, it seems to me that modern LaTeX packages usually don't support XeTeX, and it was always a quirky engine with compatibility issues, requiring special handling from both package and document authors. It's also of course missing features that are now expected by advanced users.

So moving to XeTeX would hamper relying on the rich TeX ecosystem.

[ViralBShah]

That's useful background.