Document `independent` axes scales

[dwinkler1]

Maybe I should file this upstream but it seems resolving independent axes is not well documented (see here and here). I am happy to open a PR if that is welcome. I am thinking of something like:

using VegaLite, VegaDatasets

dataset("movies") |>
@vlplot(:point, columns=2, wrap="MPAA_Rating:o", x=:Worldwide_Gross, y=:US_DVD_Sales,
        resolve = {scale = {y="independent"}})

but I am not sure this is the best/easiest way to do this.

[davidanthoff]

I think we've so far left those kind of documentation things upstream, but maybe we should start to document that here as well... Not sure.