docs: add Zero sync documentation and update existing docs

- Create how-to/zero-sync.mdx setup guide
- Update docker-compose.mdx with zero-cache service, env vars, troubleshooting
- Update dev-compose.mdx with Zero build arg
- Update install-script.mdx with zero-cache URL
- Update manual-installation.mdx with Zero frontend env var
- Add Zero sync page to how-to index, meta.json, and sitemap

surfsense_web/content/docs/docker-installation/install-script.mdx

@@ -38,3 +38,4 @@ After starting, access SurfSense at:
 - **Frontend**: [http://localhost:3929](http://localhost:3929)
 - **Backend API**: [http://localhost:8929](http://localhost:8929)
 - **API Docs**: [http://localhost:8929/docs](http://localhost:8929/docs)
+- **Zero-cache**: [http://localhost:5929](http://localhost:5929)