Document/clarify current header state in README

In v5 we have:

- an EDEK returned from the TSP with a `cmk_edek.proto` header
- a document with just `0IRON` for a header to identify an Ironcore document
- a proto that the client wraps the EDEK returned from the TSP, `icl_header_v4.proto`
- another header with the kms (or other) key id that is added by the client to the front of the wrapped EDEK

This is pretty confusing and we need to document it well (for ourselves and consumers of this library as an example of our documents).

Also indicate which TSCs / ironcore-alloy are using what.