Documenter update

[daschw]

No description provided.

[mortenpi]

I happened to notice this, I hope you don't mind some comments 🙂

• I don't think build stages are really necessary here, as there is no test phase.
• Pkg.develop(PackageSpec(path=pwd())) will not work here I think, since the repo is not really a package.
• No need for two different Project.tomls here (as it is not a package). One that sets the environment for the whole thing should be enough.

Is the goal to upgrade to Documenter 0.20? In that case, I think only the following should be necessary:

• Set [compat] for Documenter to Documenter = "~0.20" in Project.toml.
• Add DocumenterMarkdown to deps and import it in make.jl
• Update the makedocs and deploydocs arguments according to the changelog.

[daschw]

Thanks a lot for your helpful comments, @mortenpi!