Update vignette to show how to pass `igraph` layouts

[briatte]

Should happen in the 'Main building blocks > ggnetwork' section.

[micwij]

Hi, I hope it is okay to put this here. I didn't want to open an issue, just have to add a small suggestion. Maybe it is worthwhile to mention in the vignette that while "Mapping a color to both a vertex attribute and an edge attribute will create a single color scale that incorrectly merges both attributes into one" there is a possibility to work around that by setting the vertex shape to e.g. 21 and then mapping the fill argument to the vertex attribute. I think the nodes then by default have a black border but at least this allows to add different colors to edges and nodes without merging the attributes. Thanks a lot for creating ggnetwork!