Switch to less evil Documenter 0.24 method for deploying preview docs

[oxinabox]

Onced Documenter 0.24 comes out,
it will have have a standard method for deploying docs for PRs.

We can thus stop using the current hacky one.

JuliaDocs/Documenter.jl#1131

[oxinabox]

But the problem is Documentor 0.24 has very different CSS, which breaks the styling of the site.
Which is kinda nice right now.

I think a much never version still (0.25?) supports themes so that could be fix again.
But it would be effort

[nickrobinson251]

it would be effort

I took a look into doing this with v0.24 (based on how DrWatson.jl does it), and it is very painful