CI: Check intra-doc links

Credit: https://twitter.com/tomaka17/status/1176017851410526208

group/src/lib.rs

@@ -1,3 +1,6 @@
+// Catch documentation errors caused by code changes.
+#![deny(intra_doc_link_resolution_failure)]
+
 use ff::{PrimeField, PrimeFieldDecodingError, ScalarEngine, SqrtField};
 use rand::RngCore;
 use std::error::Error;