Update to Documenter v1

[oschulz]

No description provided.

[oschulz]

@fhagemann there are a bunch of errors "invalid local link/image: path pointing to a file outside of build directory in src/api.md" now with Documenter v1, not sure if Documenter has just become more picky with checks, of if this a new requirement in Documenter v1.

[fhagemann]

The problem is that the docstrings include links to images (which anyhow did not seem to work previously).
I removed the lines from the respective docstrings, let's see whether this fixes the failing tests.

[fhagemann]

I‘m a bit concerned about the fact that the docs now take longer than the tests (?) @oschulz

it used to be something like 6min in the past, now 40 min

[oschulz]

I‘m a bit concerned about the fact that the docs now take longer than the tests

Yes, that's no good. I just don't know why ...

[fhagemann]

Building the documentation in julia-1.10 for now, because it takes somewhat longer on julia-1.11