Check for broken links in CI

Add `-- -D rustdoc::broken-intra-doc-links` to the docs build in CI to
check for broken links. In order to use this flag use `cargo rustdoc`
instead of `cargo doc`.

contrib/test.sh

@@ -64,7 +64,7 @@ fi
 
 # Build the docs if told to (this only works with the nightly toolchain)
 if [ "$DO_DOCS" = true ]; then
-    RUSTDOCFLAGS="--cfg docsrs" cargo doc --all --features="$FEATURES"
+    RUSTDOCFLAGS="--cfg docsrs" cargo rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links
 fi
 
 # Webassembly stuff