docs.rs page not linked from scx_layered crates.io

[JakeHillion]

scx_layered is published at https://crates.io/crates/scx_layered, and the documentation is published at https://docs.rs/crate/scx_layered/latest. However there is no link to the documentation like most crates have. The docs should be linked from the crates page.

We only have partial documentation generated because the crate is a binary crate. If we add a lib.rs with the configuration components in these will become usable by others and also have their rustdoc generated.