Fix building docs locally

[KnutAM]

Following discourse I ran
julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()' [1]
This adds MaterialModels to the docs/Project.toml. Then, running
julia --project=docs/ docs/make.jl [2]
builds the documentation locally just fine.

So I guess [1] is only required the first time to generate the Manifest (and download packages if required) and [2] each time one wants to build the docs?

We could also include a compat section for documenter as in Fredrik's post above I guess...
(Sorry about the back and forth, did not realize [compat] was already there from before...)

[kimauth]

Suggested change

	Documenter = "0.26"
	Documenter = "0.27"

[kimauth]

I think for building the docs locally you still need to make sure you add MaterialModels with a path (either local, i.e. ".." or to this github repo), for me it complained that the package is not a registered package when I tried to instantiate.