Rename LayerSet to LayerContents & clarify ordering

[RickyDaMa]

Will close #326

Documentation also now explicitly states the layers are ordered

---

If we were interested in maintaining backwards compatibility, we could do a re-export under the old name:

#[deprecated(
    note = "old naming convention, switch to LayerContents instead",
    since = "0.12.2",
)]
pub use LayerContents as LayerSet;

[cmyr]

I'm happy either way, I'm not sure we have enough outside users to be too concerned about the breakage, but if you're feeling extra kind you should feel free, in which case I'd probably simplify the deprecation message to just "renamed to LayerContents" (but anything is fine really)

[cmyr]

Happy with or without the deprecation warning :)

[RickyDaMa]

Let's save ourselves the hassle of removing it later then 😄