Mention DisplayAs.jl in the documentation

[jtackm]

(as discussed in https://discourse.julialang.org/t/vegalite-jl-render-as-raw-image/28645)

It would be great if VegaLite.jl could automatically render alternative output formats (PDF, PNG, PDF). This can be handy for large data sets where the overhead of the default vegalite specification can be significant (i.e. when rendering inline in Jupyter notebooks) and interactivity may not always be needed.

[davidanthoff]

So I think the solution we came to in the discussion on the forum was that one can now use https://github.com/tkf/DisplayAs.jl for this. We should probably document that here, so I'm changing the title of this issue.