Documentation of `v1_layers` in COLRv1?

[v1_layer](https://docs.rs/read-fonts/0.11.1/read_fonts/tables/colr/type.Colr.html#method.v1_layer) is described as _"Returns the COLRv1 layer at the given index."_ - but it's not clear at which level in the paint graph this call is supposed to happen.

I am suggesting to either add that documentation, or - preferarbly - drop this API method. I don't think we should talk about layers with COLRv1. A PaintColrLayers table can appear as the root of the graph, but it can also appear in other locations in the paint graph as a shortcut for a chain of PaintComposites.

Since also, a PaintColrLayers interface exists that allows enumerating layers, it seems to be that v1_layer is redundant and - as described above - not 100% clear at which level it applies.

Start with the read-fonts COLR API linked for v1_layer and compare it with the PaintColrLayers interface. Determine whether the graph-level behavior can be documented clearly or whether the redundant method should be removed; done means the chosen API or documentation change is agreed and reflected in the project.

Written by the indexing model from the issue text.