Docs: Add orion::hash to root docs.
//! ## Message authentication
//! `orion::auth` offers message authentication and verification using HMAC.
//!
+//! ## Hashing
+//! `orion::hash` offers hashing using BLAKE2b.
//!
//! ### A note on `no_std`:
//! When orion is used in a `no_std` context, access to nearly all functionality
