Add `AnimationGraph::from_clips` and simplify `animated_fox` example

[rparrett]

Objective

Add a convenience constructor to make simple animation graphs easier to build.

I've had some notes about attempting this since #11989 that I just remembered after seeing #14852.

This partially addresses #14852, but I don't really know animation well enough to write all of the documentation it's asking for.

Solution

Add AnimationGraph::from_clips and use it to simplify animated_fox.

Do some other little bits of incidental cleanup and documentation .

Testing

I ran cargo run --example animated_fox.