Add CI job to prevent doc warnings

Over time, doc build warnings accumulate and only get addressed periodically. For example: #2938

Follow-up mentioned in #2938.

This issue proposed a new CI job that will fail PRs if any new warnings are introduced in a PR, to avoid manual cleanup being necessary in future.

Read the follow-up context in #2938, then inspect the repository's existing CI configuration and documentation build entry points. Define how warnings introduced by a pull request can be detected without treating existing warnings as failures; done means the CI job fails for new warnings and passes when none are introduced.

Written by the indexing model from the issue text.