Check dead doc links / API parity on CI

* This project's CI enforces a 100% documentation coverage for the library to be published on Hackage. Perhaps we could use something like https://pypi.org/project/deadlinks/ to enforce non-broken links too.
* It would be nice to enforce API parity between `Protocols.Df` and `Protocols.Df.Simple` (where possible).

Start by reviewing the repository's existing CI configuration and documentation coverage checks, then inspect the Protocols.Df and Protocols.Df.Simple APIs. Determine how dead documentation links and possible API mismatches can be checked in CI; done means both checks run automatically and fail for detected problems.

Written by the indexing model from the issue text.