Allow setting global default for `CollapsedDocStrings`

[fingolfin]

We would like to have all (or almost all) or docstring collapsed. Right now that means we have to add CollapsedDocStrings = true to every single .md file.

It would be nice if there was a way to set this once, globally, say in the call to makedocs. Perhaps like this?

    makedocs(;
        sitename = "Blah",
        format = Documenter.HTML(
            collapsed_docstrings = true,
        ),
    )

(This is somewhat orthogonal to the approach suggested in #2512 and strictly speaking we only need one, but it seems natural to me to have this, too?)