layered crate: additional feedback

[martintmk]

Sorry, I did not get to review this last week. That said, could we do a follow-up PR addressing a few issues:

• cargo doc gives me various warnings coming from layer, like warning: unresolved link to DynamicService --> crates/layered/src/lib.rs:113:40
• most tests should be moved to tests/
• the "Key Concepts" section is to short and doesn't quite make sense to me. For example, when you write "Service: An async function In → Out that processes inputs" you seem to want to say that a Service acts like a function (as it transforms some input to some output), but clearly it's a type with a trait implementation. I think all of these entries should be more clarified. I also wouldn't use the term "factory".
• The example below that ("Layers and Middleware") doesn't work out well what a middleware actually is. It seems to be left as an exercise for the reader
• Why is Execute called that? Shouldn't this be a ServiceFn or so?

Also, some docs look like so (apparently related to warnings above)

Originally posted by @ralfbiedert in #189 (comment)