Follow-up to #3513.
This removes the old blanket `std::fmt::Debug` impl on `dyn Layer` which
did not seem to be used from anywhere (no compilation errors after
removing).
Adds `std::fmt::Debug` requirement and implementations for `trait Layer`
implementors:
- LayerDescriptor (derived)
- RemoteLayer (manual)
- DeltaLayer (manual)
- ImageLayer (manual)
Manual implementations are used to skip PageserverConf, tenant and
timeline ids, large collections.
Adds and adjusts some doc comments to be more rustdoc alike.