Fix doc warnings and link failures (#1948)

Also add configuration to CI to fail doc generation if any links are
broken. Unfortunately we can't blanket deny all warnings in rustdoc
since some are unconditional warnings, but for now this is hopefully
good enough.

Closes #1947

.github/workflows/main.yml

@@ -49,6 +49,8 @@ jobs:
   doc_api:
     name: Doc - build the API documentation
     runs-on: ubuntu-latest
+    env:
+      RUSTDOCFLAGS: -Dintra-doc-link-resolution-failure
     steps:
     - uses: actions/checkout@v2
       with: