Fix pkgdown example to write plots as tempfiles and read from disk

[stefaneng]

I had updated pkgdown to print plots directly as a quick fix to render the plots. The temporary paths are still commented out in the man pages. The better way to ensure that the plots render correctly is to read the temp files from disk as a png/pdf/whatever output format and display. See this example for how to do it with PNG. Each image type is unique in how it is read from disk though:

filename <- tempfile('example_shared_colourkey', fileext = '.png');
create.multipanelplot(
    plot.objects = list(p1, p2),
    layout.width = 2,
    layout.height = 1,
    width = 10,
    height = 6,
    resolution = 200,
    legend = list(
        bottom = list(
            fun = colourkey
            )
        ),
    filename = filename
    );

# To ensure that width/height are maintained by github/reprex/pkgdown
# All this does it load and print the png we saved
grid::grid.raster(png::readPNG(filename)

See the full example here: #139