Clarify what is the public API of mlkem-native

[hanno-becker]

Currently, we only have mlkem/*, with a host of headers and compilation units. At the latest when we want to offer mlkem-native as usable as a library as well as a code package, we need a separate directory with the public-facing headers. Even when continuing to offer mlkem-native as a code-package only, such directory is helpful to communicate the API that we promise to be stable.