Add basic docs for (Lazy)Artifacts stdlibs

[fingolfin]

Some progress on #38462. Certainly could be improved but it's a start.

[simeonschaub]

Looks like the module needs to be explicitly specified in the doctests of split_artifact_slash here.

[fingolfin]

BTW in case anybody wonders why these four functions and not others: I took the Artifacts API reference in the Pkg.jl documentation and extracted all functions from there which also are in Artifacts.jl, and put them in the above list. That seemed safest: these are functions somebody already decided should appear in a public manual. I think there are a few with docstrings which should not appear here (they are rather about implementation details), but perhaps a few more should appear? In any case, I rather don't want to be the person to decide that; but I'll happily add more here if people request it.

[fingolfin]

Fixed the first doctest issue, but that now revealed an actual (?) issue in the doctest.

[fingolfin]

The doctest problem is caused by the need to double quote backslashes in strings TWICE in regular doc comments: The source code has this in the doc comment:

julia> ret = split_artifact_slash("Foo\\bar\\baz.so");

but this is of course inside a triple-quoted string, so one level of quoting is removed, so the actual content of the doc string are:

julia> ret = split_artifact_slash("Foo\bar\baz.so");

and so the \b are treated as control characters.

One fix (that I just applied) is to use \\\\ but here I chose to instead switch the docstring to a raw string because this was more legible. But if people prefer a different solution, let me know.