docs: add Gemini provider documentation

- New usage/gemini-provider.mdx with setup guide and free tier info
- Add Gemini settings to configuration.mdx
- Remove obsolete cleanup-hook.js references from docs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/public/docs.json

@@ -36,6 +36,7 @@
           "introduction",
           "installation",
           "usage/getting-started",
+          "usage/gemini-provider",
           "usage/search-tools",
           "usage/claude-desktop",
           "usage/private-tags",