Redistribute contents of 2D and 3D examples crates

[alice-i-cecile]

As the engine has grown, the 2D and 3D folders have become fairly ambiguous catch-all categories.

For the most part, these examples belong in either a rendering folder, a "stress-testing" folder, or the transforms folder.

[cart]

I think 2d and 3d are useful as categories because they are almost always "distinct". A user is either building a 2d game or a 3d game. Is it really useful to put a "3d screen space ambient occlusion" example adjacent to a "sprite" example in a general purpose "rendering" folder? The intersection between those two features for a given project is "empty". Someone looking for a way to draw a sprite is probably more likely to search in a "2d" folder than a "rendering" folder.

[alice-i-cecile]

Yeah; this ties into some of the broader conversations I've had with @mockersf about "how do we organize and scale examples". Both organization methods can be useful, and we need a way to address organization of the growing list of examples without simply stopping adding them.

[mockersf]

I think we should just give up on examples organization / discoverability in the GitHub repo: bevyengine/bevy-website#299

(and by give up, I mean try to keep it simple, but it's not the right place for that. it should be on the website, and the repo example folder should be more a concern for Bevy developers, not Bevy users)

[alice-i-cecile]

Sounds good to me. We can do a better job of it there.