sphinxdocs: output github actions workflow command compatible output

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.

Start by reviewing the GitHub Actions workflow-command format linked in the issue and how the Sphinx documentation build exposes warning and error output. Determine whether a Bazel flag or Sphinx plugin is the appropriate entry point; done means documentation build messages are emitted in a format that GitHub can display in the code review UI.

索引モデルが issue の本文から書いたものです。