update to Documenter 0.25

[simeonschaub]

This deletes our old hacky workaround for PR previews and replaces it by Documenter's own push_preview version, that was added in 0.24. Hopefully, we should see something at https://www.juliadiff.org/ChainRulesCore.jl/previews/PR200/.
Closes #153

[nickrobinson251]

Thanks!

Unfortunately this seems to break the custom CSS, including the styling of the "terminology" admonitions. It'd at least be good to fix those of possible

[simeonschaub]

Oh right, didn't notice that at first! Seems like Documenter changed some stuff around internally. I will see, what I can do.

[oxinabox]

This will close #153

Like Nick said this breaks the custom CSS.
Which is why i didn't upgrade before.
but now it is possible to use the themes stuff to customise the CSS again so we should be able to make it work

[sethaxen]

Note also that recent versions of Documenter by default use KaTeX to render equations, which renders faster but does not support as much as MathJax. In particular it breaks the array rules tutorial: https://www.juliadiff.org/ChainRulesCore.jl/previews/PR200/arrays.html. There's an argument to switch the backend back to MathJax, or we can update the tutorial to only use KaTeX supported features. I'm inclined to say we do the former for now and then revisit the latter in another PR if we wish.

[simeonschaub]

The dark theme doesn't look quite right with our modifications, so I still need to figure out how to fix this.
@sethaxen JuliaDocs/Documenter.jl#1367 should be merged soon and my goal is to eventually use this with the physics package in ChainRules, so we don't always have to write \frac{\partial f}{\partial x} for every partial derivative. We could also make our differentials look a bit nicer – I personally am a big fan of non-italic d, but it's a bit of a pain to write in bare LaTeX. MathJax3 should also load almost as fast as KaTeX.

[oxinabox]

For anyone else who wants a quick side-by side.
This is the current status.
New is on the left, old is on the right

Looking good!

[simeonschaub]

Should be good to go now. I have disabled most of our customizations for the dark theme, except for our methodology blocks. If we really want to customize the dark theme as well, we could always do that at a later time.

[oxinabox]

Looking at this on my phone.
In Dark Mode
Gorgeous.

[simeonschaub]

Weirdly, the yellow for the Terminology box looks a lot whiter on your screenshot than it does for me. That's probably just a color calibration thing and not our fault though. My browser just displayed the screenshot wrong.

[simeonschaub]

Ok to merge?