Add docs/source/companion.rst: what the feature is, why/when to use it,
quick start, per-companion options, states, runtime control via the
socket and gunicorn-companion CLI, reload/shutdown behavior, and
limitations. Wire it into the toctree and the feature list on the index.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
