Docs: Re-add crate-level doc about kdf
//! ## Password hashing and verification
//! `orion::pwhash` offers password hashing and verification using PBKDF2.
//!
+//! ## Key derivation
+//! `orion::kdf` offers key derivation using PBKDF2.
+//!
//! ## Message authentication
//! `orion::auth` offers message authentication and verification using HMAC.
//!
