Setup a docs CI

[kellertuer]

I was not so sure how to best help here.
I think the open issues and their details are great, but I do not have enough experience in distributions.

So here is what I think I can help with.

• setup a CI to render the docs
• extend the make.jl to
  • also run locally as a script
  • specify in the docs/ environment that ManifoldDistributions.jl is always the one from the parent dir
  • add DocoumenterCitations so the can [key](@cite) literature from the references.bib
  • add DocumenterInterLinks so we can e.g. [`Rotations`](@extref) to the docs of other packages – for now I added Manifolds.j and ManifoldsBase.jl

I hope this is a nice small PR – and if I see further ideas I hope to be able to contribute more, since I like the idea of this package a lot.

[kellertuer]

Oh, I hab hoped this would take up the GitHub orgs secrets, maybe we need new local secrets here still?

[kellertuer]

Ah, yes, as for so often the documenter key/secret was missing, now we have a first deployment with a preview at

https://juliamanifolds.github.io/ManifoldDistributions.jl/previews/PR9/