docs(deploy): update README and .env.example for Docker deployment

- Refresh README feature list and remove references to the legacy
  in-app monitoring charts / backend poller.
- Document configurable dashboard widgets, addon pages, and widget env vars.
- Add VITE_PROMETHEUS_URL support to frontend Dockerfile and both compose files.
- Add header comment to .env.example explaining shell-export workflow.
- Update remote server requirements to match current capabilities.

docker-compose.dev.yml

@@ -40,6 +40,7 @@ services:
       VITE_OIDC_ENABLED: "false"
       VITE_DEV_API_PROXY_TARGET: "http://backend:8000"
       VITE_GRAFANA_URL: "http://localhost:3000"
+      VITE_PROMETHEUS_URL: "http://localhost:9090"
     ports:
       - "5173:5173"
     volumes: