Binder setup incorrect

[devmotion]

Currently, binder does not recognize that the notebooks use Julia and therefore does not install the correct kernels. It seems Project.toml and Manifest.toml files have to be added in the gh-pages branch (see https://mybinder.readthedocs.io/en/latest/examples/sample_repos.html#julia-binder-demo and fredrikekre/Literate.jl#46 (comment)). If the examples (currently there's only one) require different dependencies, probably it would be helpful to add separate Project.toml + Manifest.toml as suggested in JuliaGaussianProcesses/KernelFunctions.jl#234 (comment).

[devmotion]

Unfortunately, it seems it is not possible to use directories with different project environments (see https://mybinder.readthedocs.io/en/latest/using/config_files.html#project-toml-install-a-julia-environment and jupyterhub/repo2docker#1002). Project.toml (and possibly Manifest.toml) should either be in the root directory of the branch or in a "/binder" subfolder.

[devmotion]

So it seems one has to add one specific set of Project.toml + Manifest.toml files to the root directory of the gh-pages branch. Of course, we could just always copy the current setup in the docs folder (as long as examples don't use their individual environments). However, it seems then always all notebooks, including the ones in the stable subdirectory or the ones in the documentation of previous releases, would use the same project environment. Probably, this will break some of the previous tutorials...

Maybe we should just remove binder links? Or use a different branch for each tutorial?