Viz tutorial tweaks

[hyanwong]

As discussed in #164 it might be more helpful to put the red "tree branch" colouring example into the list of examples at the end of the viz tutorial, rather then having lots of popgen explanatory text in the middle of the viz descriptions.

Also, we should update the ARG example to use networkx and graphviz, as we are doing in the ARG paper, perhaps using the example of the Wiuf and Hein ARG. Both changes can be done at the same time, I reckon. (replaced with the nice tskit_arg_visualizer)

[petrelharp]

Also, the SVG looked good separately, but is rather small in the docs:

[hyanwong]

It might look better on a wider screen?