Document that the Postgres user needs REPLICATION and CREATE privileges
for zero-cache. Docker setup handles this automatically (superuser), but
managed databases (RDS, Supabase, etc.) may need explicit grants.
- 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
