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.

首先查看 issue 中链接的 GitHub Actions 工作流命令格式，以及 Sphinx 文档构建如何暴露警告和错误输出。确定 Bazel 标志还是 Sphinx 插件是合适的入口点；完成的标准是文档构建消息以 GitHub 能在代码审查界面中显示的格式发出。

由索引模型根据 Issue 内容生成。