fix(identity): build with all features on docs.rs

Resolves #3810.

Pull-Request: #3828.

identity/src/lib.rs

@@ -33,6 +33,8 @@
 //! (e.g. [ed25519 binary format](https://datatracker.ietf.org/doc/html/rfc8032#section-5.1.5)).
 //! All key types have functions to enable conversion to/from their binary representations.
 
+#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
+
 #[cfg(any(
     feature = "ecdsa",
     feature = "secp256k1",