CI: Check intra-doc links

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

zcash_proofs/src/lib.rs

@@ -1,3 +1,6 @@
+// Catch documentation errors caused by code changes.
+#![deny(intra_doc_link_resolution_failure)]
+
 use bellman::groth16::{prepare_verifying_key, Parameters, PreparedVerifyingKey, VerifyingKey};
 use pairing::bls12_381::Bls12;
 use std::fs::File;