Fix CI issues with Julia nightly caused by Markdown stdlib changes

[fingolfin]

I made a bunch of changes to the Julia Markdown stdlib, and looking here it seems they caused some Documenter tests to fail. For the ones I looked at, the Documenter tests should be adjusted, but of course it's also possible that there are cases where the Markdown.jl changes are buggy. Either way, it needs to be addressed.

I'll try to take care of this in the coming days, but in the meantime wanted to log it as a known issue.