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.

.env.example

@@ -1,3 +1,7 @@
+# Manage environment template
+# Copy this file to .env, fill in the required values, and export them in your shell
+# before running docker compose. Compose files use interpolation, not env_file.
+
 # App
 APP_VERSION=0.1.0
 APP_BUILD_INFO=dev
@@ -44,6 +48,7 @@ VITE_OIDC_REDIRECT_URI=https://manage.example.com/oidc/callback
 VITE_OIDC_POST_LOGOUT_REDIRECT_URI=https://manage.example.com/
 VITE_DEV_API_PROXY_TARGET=http://backend:8000
 VITE_GRAFANA_URL=https://grafana.example.com
+VITE_PROMETHEUS_URL=https://prometheus.example.com
 
 # SMTP
 SMTP_HOST=smtp.example.com