Update AnimatableProperty documentation, reduce crate dependencies

[greeble-dev]

Objective

• Remove the second to last bevy_animation dependency on bevy_render.
• Update some older documentation to reflect later changes to the crate.

Narrative

I'm trying to make bevy_animation independent of bevy_render. The documentation for bevy_animation::AnimatableProperty is one of the last few dependencies. It uses bevy_render::Projection to demonstrate animating an arbitrary value, but I thought that could be easily swapped for something else.

I then realised that the rest of the documentation was a bit out of date. Originally AnimatableProperty was the only way to animate a property and so the documentation was quite detailed. But over time the crate has gained more documentation and other ways to hook up properties, leaving parts of the docs stale or covered elsewhere. So I've slimmed down the AnimatableProperty docs and added a link to the main alternative (animated_field).

I've probably swung too far towards brevity, so I can build them back up if preferred. Also the example is kinda contrived and doesn't show the range of AnimatableProperty, like being able to choose different components. And finally the memes might be a bit stale?

Showcase

Testing

cargo doc -p bevy_animation --no-deps --all-features
cargo test -p bevy_animation --doc --all-features