refactor: Use extended key value attributes feature instead of external doc

See:
 - rust-lang/rust#83366
 - rust-lang/rust#44732
 - rust-lang/rust#82539

Author: jhg

src/lib.rs

@@ -1,5 +1,5 @@
-#![feature(external_doc)]
-#![doc(include = "../README.md")] // This also allow to run examples in that file.
+#![feature(extended_key_value_attributes)]
+#![doc = include_str!("../README.md")] // This also allow to run examples in that file.
 
 #![allow(unsafe_code)]
 #![warn(missing_docs)]