sphinxdocs: output github actions workflow command compatible output

[rickeylev]

GitHub actions support specially formatted output that can then show up in the ui: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions#setting-a-warning-message

So the idea would be: have a bazel flag and/or sphinx plugin that transforms warning/error messages that occur during docs building into that format. They can then show up in the code review UI.