Better surface warnings from the docs build as warnings in GHA

This ought to be possible by telling GHA which patterns to look for as warnings, I forget the actual command syntax, will fill this out later, but jotting down now so I don't forget.

Start by reviewing the GitHub Actions workflow and documentation build configuration, then check GitHub Actions syntax for marking matching build-output patterns as warnings. Done means warnings from the docs build are surfaced as warnings in the workflow logs or annotations.

Written by the indexing model from the issue text.