In the doctests in kem.rs, the actual tests that are run to verify that the KyberKem and the DummyKem actually work
are wrapped inside a function to make use of the ?-operator. However, these functions were never called and thus
the tests weren't really helpful and didn't provide proper coverage.
Some links in the documentation of the ciphers and cipher-traits were broken or linked to private fields.
This PR fixes most of these occasions and some more warnings in cargo doc.
The reaming issues are links to chacha20poly1305_ietf, that are broken because the feature experiment_libcrux corresponding feature is enabled. Analogously, disabling the feature would lead to broken links to chacha20poly1305_ietf_libcrux.
