Add HTML cross-reference to compare library behaviour against

[TheBeardedQuack]

For each example, I aim to create a HTML doc which uses a <div> for each bevy Node, and produces the same UI from the same CSS source file.
These HTML and Rust versions need to be kept up-to-date together if the structure ever changes, however CSS changes are intended to be reflected in both without code-changes.

If library version X produces one image, and library version Y produces another, at the moment there's no way to reason which is "correct". With CSS originating from the web, once created, these HTML examples will provide the "source of truth" reference to evaluate how accurately the library is representing the scene.