[web-animations-2] Update the GroupEffect sections in level 2 spec

[yi-gu]

We have started looking more closely at GroupEffect (explainer, prototype). As such we want to ensure its specification moves forward as well as part of [1].

In this PR we made some fairly minor changes to the current design. PTAL. Thanks!

[1] #4008

[birtles]

I tried to build this PR locally but got a bunch of link errors. Is it supposed to work?

Also, any chance we could replace those PNG images with SVG images in a similar style to the existing ones? (Preferably not the UML class diagram style.)

I'm not totally convinced that StaggerEffect is necessary. It seems a lot of stagger-type effects use a decreasing delta (e.g. 0s, 1s, 1.8s, 2.5s, 3s, 3.3s, 3.4s) or use some degree of random jitter in the amount of stagger. It would seem like script can easily generate a ParallelEffect to achieve these cases and the original StaggerEffect case.

The link to the explainer at the start of this thread also appears to be broken.